In regards to forgetting to mention a mortal sin in the confessional:
As long as you confess every unconfessed mortal sin that you’re aware of, you’re fine. If you forget to mention a mortal sin in confession, it is forgiven. However, if you remember it later, you have to bring it up next time you go to confession (assuming you don’t forget it again).
The idea with confession is that as long as you don’t hold anything back (in terms of unconfessed mortal sin), God holds nothing back in terms of His mercy. If you forgot something, then your confession is still valid, since you didn’t willfully hold anything back. Afterwards, if you remember an unconfessed mortal sin from the past, then mention it next time you go to confession (whenever that may be), so that, again, you’re not holding anything back. However, the previous confession is still valid, so there’s no need to rust off to confession; you’re still in a state of grace. Just mention it next time you go.
In terms of what happened during the confession you described:
As you mentioned, you didn’t know what was from the past and what was current, so it’s fine that you just read through everything. As a rule of thumb, if you think you suffer from scrupulosity (something that I’ve dealt with for years), I’d strongly advise against keeping lists of sins. When I did that, it caused me to become obsessive about past sins and led me to make ridiculously exhaustive lists that took forever to confess.