So you're assuming internal forum guilt of a mal-formed conscience.  Got it.  Most people growing up in the NO don't now anything else.  I grew up thinking simply that this is a Catholic Mass.  I could discern bad practices in the NOM, in which I would refuse to participate, i.e. Communion in the Hand, etc., but that's it.  I was nearly 30 years old before even the internet was a "thing".

I wrote quite clearly above, that that there could be some culpability (known in most cases only to God) with regard to whether or not the individual sufficiently informed himself.

To extend my example of the $100 bill above.  I take $100 off a table (and pocket it), thinking it's mine, though in reality it belongs to someone else.  Maybe I should have investigated, or asked around first, but heck if I didn't just have a very similar $100 bill a few minutes earlier, so it never even occurred to me that it just might belong to someone else rather than being my own.

BOTTOM LINE:  You cannot commit a grave sin without knowing it to be a grave sin and willing it anyway.  Nobody commits a grave sin without knowing it.  This is utterly absurd and people have to stop trying to spread that crap. Natural law is known in written in men's hearts and is knowable there, just like the existence of God, and the only way one doesn't know it is by drowning it out.  But positive law, such as the requirement to fast on Fridays, or questions like whether the NOM is displeasing to God, those are not.  Very many sincere individuals have concluded that the NOM is not offensive to God.  I could sit here myself and make a convincing devil's advocate case for the NOM myself, reducing the evils we see to "abuses" of the "pure" NOM.

There's literally no reason not to do so. People should be making yearly general confessions anyway, but instead they're reluctant to do it once after a supposed conversion from a lax Novus Ordo life. Ridiculous.

General confessions should be made:
1) before receiving the sacraments of confirmation and matrimony
2) at any important spiritual junction or when turning over a new leaf after a period of sin
3) periodically (St. Francis de Sales says yearly) to reflect on your past life and ensure the validity of your confessions.

Priests should recommend general confessions regularly instead of thwarting them.

In a Quodlibetal question St Thomas says, 

"sometimes an erroneous conscience does not absolve or excuse from sin, namely when the error itself is a sin, proceeding from ignorance of that which someone is able to and obliged to know, as for example, if someone believed fornication to be simply a venial sin, and then, [if he committed fornication], although he would believe that he was sinning venially, he would not be sinning venially, but mortally" (Quodlibetal 8, q. 6, a. 5)

I fear that I will become scrupulous if I make general confessions more, so far I have only done it once since my conversion, and everytime I remember a past mortal sin I make sure to confess it at the next confession.

For the anonymous who quoted "the new CCC":

This is not a real Catholic catechism. This is the Novus Ordo catechism. People won't take your arguments seriously if you quote Modernist books, and you take a serious risk of learning some grave errors by reading this horrible books.

Be aware of Modernism.

If I forgot it was Friday and had ham for breakfast did I commit a sin?