Are they sinful? Are they mortally so? To be clear I'm asking about swearwords other than the Lord's Name in vain.
I once heard a priest say that swearing was a mortal sin but I wasn't given a clear source on why.
I don't know why someone would down thumb your two question, so I up thumbed them to balance things.
If a priest said that swearing is a mortal sin, then I would stop swearing and confess my mortal sin. No big deal. You are not going to find me spending even one second finding out what the Moral Theology Books say, I could care less. It is not like he is asking me to cut off my arm.
To answer your question, when I am in doubt about a mortal sin, I go and confess it as a mortal sin, and let God sort it out. And that's the last thought I give it.
"Kill them all, let God sort them out"