This is not correct.  The first general confession must be for sins committed since attaining the age of reason or since baptism.  Any general confessions after that should begin with sins committed since the previous general confession.
You cannot just "pick and choose" during a general confession.  Otherwise, this is simply a normal confession where you are confessing some past sins.

So, with this example, if your "bad-mouthing" involved calumny or detraction, then that would be detraction and calumny.  But if it's something short of that, such as "boy he gives lousy sermons" or "that priest has a bad temper" or "I really don't like him." ... which is typically what is meant by bad-mouthing rather than "this priest is having an affair with a female parishioner".

I think it depends on whether you had good reason for speaking that way.  I think it maybe be possible to "criticize" a priest in order to alert others to issues they might expect from him.  If the motivation was just idle gossip or a desire to vent, then I would think it to be a venial sin.

We've had similar discussions about this type of thing in a different context.  If I ran into a priest who came across as effeminate and giving of a "pedophile" vibe, I think that it would not only be permitted but even required to sound the alarm.  I think that too many pedophiles got away with their crimes precisely under cover of scruples regarding not saying anything bad about a priest.

It sounds like you are just expressing your opinion, I don't see it as sinful unless your are intentionally going around bringing up the subject in order to damage his reputation.  
He may be treating you like he is because he thinks it's necessary, because of your scruples, to do so. Ask around, discreetly, to see if anyone else is having the same experience.

Is it a sin to use pseudonyms on social media?  

Here's a question that I hadn't considered until my last confession this past Sunday that I'm seeking a bit of clarification on.

If someone with scrupulosity is genuinely concerned that they remembered that there was a major past sin which they had not previously confessed and they felt the need to go to confess it elsewhere in order to make sure they had done their part to truly complete the act of reconciliation with God but have been advised by their confessor to refrain from doing so or making general confessions but they do it anyway out of genuine concern is that a major sin?

Despite my confessor's directions to not do this, sometimes my scrupulosity has gotten the best of me to the point where I have gone to different priests to either confess certain past sins that I have forgotten or do general confessions. So far, I've made a total of four general confessions; three of which were after my confessor advised me to not confess any past sins. Each time, I had been concerned that the previous one had been flawed in some way due to having misremembered or forgotten something for the fear of possibly not having had true contrition or a firm purpose of amendment. On one occasion, I also went elsewhere to confess a few that I had forgotten or previously misremembered in addition to any sins committed since my previous confession or ones that I was unsure about.

I've always read (though this is obviously directed at people who don't suffer with scrupulosity) that if you do forget a past sin you should confess it the next time you go to confession to complete the act of reconciliation. So that is a driving force behind the desire to confess past sins regardless despite my confessor saying to do otherwise. The issue, as I see it, not so much of not trusting God or the priests but, rather my own faults and mistakes in trying to make the confessions. I'm basically second-guessing whether or not I had true contrition and a firm purpose of amendment or if I misspoke and altered the meaning of what I was trying to confess so that the priest might have misunderstood what I wished to say without giving too much unnecessary information or otherwise.

For some background as to understanding why I'm dealing with scrupulosity, I know a good deal about basic, ontological theology and the tenants and history of the Catholic faith, moral theology was never something I really indulged in and, quite frankly, neglected. I'm actually quite new to the SSPX and going to the Traditional Latin Mass as I had been a "home aloner" for quite a while. I'm certainly no expert in moral theology and I'm still becoming accustomed to attending the Mass as, prior to this, my only experience was attending the Novus Ordo when I was a child. As you would expect, I was poorly catechized in my CCD classes which were taught by average, suburban mothers of children also making their first communion. I didn't even really understand the Trinity or what the Eucharist meant when I made my first communion in the Novus Ordo. I actually had to do my own research years later when I started taking religion seriously in order to actually start to understand what the Catholic faith really was.

Regarding the priests that I've talked to on the matter:

1. My confessor has told me to avoid any past sins, even ones that have not been confessed before. I can't recall if he's explicitly mentioned general confessions but I assumed that would be included anyway. I've really tried to adhere to his directions but it's very difficult.

2. The second priest who I talked to on this matter that heard my third general confession was very patient and understanding. He told me that it might be a good idea to go to a retreat because they will teach you how to make a general confession there. So he didn't outright forbid me from ever making one again.

3. The third priest who address this matter with me was actually the one who heard my second general confession which I ended up rushing through because he wasn't very patient or understanding. Following my third general confession, mentioned above, I had remembered that I forgot a few things or misremembered them and wanted to rectify the situation so, the following week, considering that it was only a few things which I needed to confess that I had forgotten the previous week in addition any sins committed since that point, I went back to this priest (who I've only met twice keep in mind) and he had zero patience and, quite honestly, made me feel intimidated by grilling me with questions implying that I was wasting his time so, as a result of that, I may have, out of panic, accidentally lied or was less than honest about. That could just be me second-guessing myself but, considering that if I genuinely had done so that would have been a sacrilege I didn't want to take any chances on receiving communion after that confession out of fear of committing yet another possible sacrilege. Anyway, to get to the point he basically told me "no more general confessions!" As a result of this last interaction, and feeling like I couldn't explain myself to my regular confessor, I decided to see if I could reach out to a different priest to would be willing to hear a fourth general confession.

4. I was lucky enough to find one who like the second priest that I previously mentioned was very willing to hear my general confession, was very very patient, kind and willing to help me along if I was stumbling, lost my place in my notes, or was naming of things that weren't actually sins. I mentioned to him that I was concerned that I had disobeyed my confessor "and other priests" (referencing the third priest above). He just told me to tell my confessor next time I go to confession that I made a general confession with him but didn't elaborate any further. I assume this is to make satisfaction of some sort since he did give me absolution. This makes me think that I should probably also explain the other times with the that I've not heeded his advice. Would that be necessary, optional, or just superfluous?

Now, since my confessor has at times gotten frustrated with me while however still being kind about it, as he knows that I struggle with anxiety, I'm concerned that because of all that I mentioned above regarding having disobeyed his directions on not confessing any past sins and going out of my way to make several general confessions, this might cause him to lose his temper with me, despite my sorrow, and might not give me a chance to explain what happened or possibly kick me out of the confessional and bar me from absolution thereby (hypothetically) making my situation worse than it was before. How likely do you think that would be?

I found a PDF file called "The Ten Commandments" for the Scrupulous" which, addressing this issue, advising scrupulous people to obey their confessors when they tell them to not repeat general confessions but, also says, "It’s most certainly not a sin to choose not to follow his directives, but it’s counterproductive and not at all helpful." So, considering that, it sounds like it wouldn't be, on the surface, a huge deal but, again, this is a rather unique situation.

I'd also like to clarify that my main concern for feeling such a need to make so many general confessions in order to do it "just right" is:

Firstly, my desire to not commit any sacrileges by making any bad confessions or receive Holy Communion unworthily. Secondly, my fear of dying in a state of mortal sin and, thirdly, my fear of what is happening in the world, how it is looking like it will begin to get far more draconian, with further lσcкdσωns due to the "variants" (which would prevent interstate travel and therefore my ability to go to confession and attend Mass) as well as the possibility of major catastrophes happening such as the power grid going down (which is looking like a strong possibility if you've been following the stuff with the pre-planned cyberattacks), the door-to-door vaccine coercion, the talk of "CÖVÌD isolation centers" for those who do not want to take the vaccine, the beginning stages of what looks like a secret police force, and the likelihood of martial law being declared if governments start to begin liquidating political and religious dissenters NKVD-style in these isolation centers and then telling everybody they died from CÖVÌD. I know this may sound extreme or paranoid but, I'm sure a lot of people felt that things like that could never happen in Russia before the Bolshevik ʀɛʋօʟutιօn due to normalcy bias. Basically, I'm very concerned that with all the stuff going on in the world many of us could be facing martyrdom and, considering that these things really aren't out of the realm of possibility as it looks like we enter the last days, I would like to make sure I've made the best confession I could possibly ever give.

So, in summary I guess my question is, considering all of the above, how should I go about informing my Confessor that I made a general confession so as to fulfill what I was told to do by the last priest to heard my general confession? How would you recommend that I go about explaining the situation to my confessor as clearly and concisely as possible without having to be as thorough and detailed as I am in this post so as not to take up any unnecessary time from him time to hear others' confessions?

If someone could give me an outline of what I should say, I would greatly appreciate it because I feel like I'm going to be tongue-tied. Thanks.

Print out what you wrote above and give it to him. When he tells you not to confess past sins actually obey him, otherwise this will never end and you will not advance in the spiritual life.