If you forget to confess a mortal sin you are required to confess it at your next confession, but you are absolved of that sin even though you forgot.
Also, you don’t need to get all that detailed. For instance you could say perhaps that you committed many acts of impurity for twenty years including premarital sex, contraception, pornography… and that could be all the priest wants possibly but you may want to ask if he wants you to go into more detail. Since acts of impurity includes a wide range of sins then even if you forget to mention something specifically then it still covered I think.
Also you could perhaps say that you were uncharitable/mean to people many times instead of saying you had an argument with your mother, you flipped off someone on the highway, you cussed at cashiers/family members.
Ideally you should say sort of what you did, but if there is not much time saying “I was impure many times” technically suffices if the priest is okay with it. It is important to have the type of sin (even if vague) and the number of time committed (even if approximate or just saying “many” or “sometimes”).
I would recommend that you look at an examination of conscience which can easily be found on the internet. Then just kind of put your sins into groups and see how you can kind of sum things up so it doesn’t run too long. Also, it would be ideal to schedule an appointment for confession if you think it will go a long time. Priests are usually very understanding and know that you wish to live in accord with the will of God since you are going to confession so you shouldn’t be worried about them judging you if that is something that you are worrying about.