I went to confession today but in the evening I remembered sins I didn’t confess (I had forgotten about them completely!) but I remember that when I comitted those sins I asked God for forgivness. The problem is that I don’t even know if they are mortal sins. I know there is 3 conditions for mortal sins but I don’t even know if the sins I committed are of a grave matter. Can I still take the Eucharist next sunday? Even if there is a chance its a mortal sin? or do I have to confess it first?
As long as you did not keep it on purpose, all your sins are forgiven even the mortal sins. However you must confess them in your next confession, and it’s better to do it as soon as you can so you wont forget them again. Tell the priest you’d like to confess sins you forgot to mention the last time. It would be better if you do it before next Sunday though it does not effect your state of grace as long as you are determined to confess them on your next confession.
As for when a person is not sure whether their sins are mortal or venial, the rule of thumb is if you are not sure then it’s a venial sin, yet the Church highly recomends us to confess these anyways, if only just for the peace of mind. The Church does recomend us to go to confession frequentaly even if just to confess venial sins. The more we go to confession the more we grow closer to holiness.
People do not normally “forget” mortal sins. You might schedule some time outside the confessional for some spiritual advice as you may need some counseling as to what is or is not a mortal sin.
And if you seem to have this problem, then get one of the many guides to an examination of conscience and use it before you get to the confessional.