Its perfectly ok, if they are mortal and you forgot them just say them in the next confession: you can tell the priest that you forgot them or not. It's up to you, I usually tell him that I forgot them last time. Well, the three conditions for a mortal sin are: it is a grave matter, committed with full knowledge, and complete and deliberate consent. As a confessor once told me when I confessed a sin but wasn't sure if it was mortal; "If you don't know if it is mortal or not, its venial." He seemed almost amused at my query.
Edit: The devil likes to play head games with you. I am not overly scrupulous, but I sometimes ask myself: did I actually sin? If you didn't make a conscious act of the will and consent to the sin, you didn't sin. Simple as that.