If you remember it later, it's ok to still go to Holy Communion--it's been forgiven, but you HAVE to confess that you forgot one at your next Confession.
Thank you for your post, it was very helpful. Thanks to everyone else as well. It is very much appreciated.
However, I have a question in relation to the above. I understand that if you accidentally forgot something in confession, then you should confess it the next time but, my concern is with remembering past mortal sins, whether relatively recently (say a few months ago) or in the distant past. These are mortal sins and I feel compelled to confess them but, my confessor tells me not to confess anything from the past. This is what really is upsetting my scruples. I feel like since, when I try to confess anything from the past and the priest basically shuts me down and says "nothing from the past!" I'm therefore omitting mortal sins (albeit not intentionally; in fact, I strongly desire to confess them). I just want to confess everything I can remember so as to complete the process of reconciliation with God.
I feel like because I'm unable to confess them, they are still with me and I'm not fully absolved or, perhaps, out of trying to be obedient to the confessor and not trying hard enough to explain the situational context, I'm therefore making a sacrilege or something along those lines.