Let’s face it, if you have a memory (and habits) like mine you’ll never remember all of the sins you’ve committed. What happens if I forget to confess a sin in the confessional? Is it forgiven?
Yes it is, IF you really forgot it. However, if it is a mortal sin then you should confess it if you remember it later on. God isn’t going to condemn you for legitimately forgetting something. However, if, and when, it comes back to you, you are obligated to confess it.