Today I was thinking about how I am determined and promise never to commit a mortal sin again and then I had these thoughts that even though I say that, I do still have the capability/free will to do so.
But by admitting that I’m capable of mortal sin, I felt like that was sinful even possibly mortally so. I’m an overthinker and I felt like thinking those thoughts was shaky ground.
If you genuinely believe this to be sinful, then I encourage you to speak to a priest:
#1: If you believe you have committed a mortal sin you are obligated to confess it, even if it isn’t actually a mortal sin by Church standards. This is done out of obedience to God, and out of our recognition that we do not want to die outside of His friendship.
#2: There’s a chance that you may have scrupulous tendencies, which the priest will probably mention after your confession. Scrupulosity can be devastating, and if there’s a chance you may suffer from it then you should talk to a priest about it.
It is not sinful to admit it, because it is true. Admitting that you might do evil helps choosing good. So you can, technically, commit mortal sin. But Saint Paul said we are free to do everything, but not everything we do is good.
Having free will, admitting or recognizing it is synonym of liberty, but not of right to do anything you want, except if you want to do good.
I don’t see that as sinful at all. On the contrary, it is a humble person who admits their weaknesses. All humans suffer from concupiscence. This tendency to sin is a weakness. Thinking that we could never commit a mortal sin could lead us to pride, which can lead to carelessness and later sin. It is wrong for a person to imagine committing a mortal sin in detail but perfectly fine to see concupiscence within themselves. Just because we have a weakness, though, doesn’t mean it can’t be improved on. We all are called to constantly strive to become closer to God. Part of that is overcoming concupiscence.