Paris,
Remember that to commit mortal sin is to commit a grave sin, with full consent, knowing that it is gravely sinful.
So, if later on you realize that something you did before was gravely wrong, it is still not mortally sinful because you did not know it was gravely wrong when you did it. If you went and did it again, though, it would be a mortal sin, because now you recognize the sinful nature of the act.
Basically, if you have any doubt whether something you did earlier is mortal, it is not.
HOWEVER, you should still confess these things, as they are still sinful (though not mortally sinful), and sacramental confession will help you overcome them and strengthen you against committing them in the future. Even so, we are forgiven the debt we owe for our sins every time we ask for it, and that debt is totally forgiven.
It helps me to think about it in monetary terms:
Say I owe God a large sum of money in payment for choices I’ve made. Although I’m not sure of the exact amount I owe, I know it’s big because I know of the major things I’ve done to incur the debt. When I go to God to ask Him to forgive the debt, I don’t just say “Forgive me this part, and that part, and this part”. Rather, I ask the whole debt to be forgiven, citing as much of the debt as I know so that God sees I understand how I incurred the debt to begin with. God honors that and forgives my whole debt. When I leave the confessional, my debt has been completely forgiven – paid back, as it were, by Christ Jesus Himself. At that point, I “owe” God nothing. When I again sin against Him, though, my debt begins again, and it is through seeking forgiveness that it is again repaid by Jesus.
Does this help?
Also, as so many others have said, our debt to God is forgiven whenever we ask Him, even ouside the confessional, provided the sin is not mortal. As I pointed out earlier, if you have committed mortal sin, you would know it because you must know it in order for it to be mortal. So seek His forgiveness regularly and humbly, and receive the Eucharist whenever you can so it can strengthen you against the enemy.
Peace,
javelin