If you can’t remember doing something that was so serious that it was a mortal sin, then it is unlikely that it was actually a mortal sin in the first place.
In any case, you can only confess what you actually remember. Anything else you’d run the risk of telling a lie to God. Which isn’t wise.
Provided your confession is as complete as you can make it, then all your sins are forgiven, even the forgotten ones. It’s a done deal at that point.
If you subsequently remember a mortal sin, it doesn’t undo the absolution, but for the sake of good form and being able to receive the advice of the priest you should remember to add it to the list next time. But there’s no hurry about it unless you go on to commit a NEW mortal sin, since your state of grace is fully valid.
