Sir Knight, I think you may be hung up on the difference between material and formal sin.
Material sin is the bad action in and of itself - always bad. Say I give you a hearty slap on the back in greeting, not knowing that you have a bad sunburn. That slap is extremely painful in and of itself because of your burn. No other factor will make it less painful. That’s material.
BUT, I don’t know you have a sunburn, and I’m just being friendly when I slap you on the back. So you are not angry with me, because I had absolutely no intent to hurt you. It is not, therefore, a formal sin.
Switch up the situation a little. I think you have a sunburn, and, wishing to cause you pain, I slap you on the back. In fact, one of our other friends is burned, not you, so the slap causes you no pain. No material sin.
BUT I intended to hurt you, and if that fact came to light, you would be quite upset with me, knowing that I wanted you to suffer. That’s formal.
So a mortal sin must be a formal sin. You must commit it with full knowledge of its gravity AT THE TIME. It wouldn’t be fair to blame you if you found out later, would it? You can commit a material sin without incurring guilt if you are truly uninformed.
Betsy