I just did this last night. My last confession was 18 years ago. I had all the same questions you did. I couldn’t remember how many times or even details. I read some questions for the examination of conscience and knew I did some of them even if I couldn’t remember details (i.e. told a secret, disrespectful to parents).
I’m not really sure what is and is not a “grave matter” and without all the details I couldn’t evaluate if what was and wasn’t venial vs. mortal. Father explained to me that part of his job was to help me evaluate what really is and really isn’t a sin that needs to be confessed. He also told me I could just say “many” if I didn’t remember the times but knew it was more than once.
I gave more of a summary and then add some specific details as I remembered them. I don’t know if my examples are the “proper” way to do it but this was how I said some things:
For ____ years I questioned my faith in God and in the teachings of the Church. As a result during those years I failed to meet my Sunday Mass Obligation and I failed to go to confession once a year. I also failed to pray and read the scripture daily.
Many times, I was disrespectful towards my parents, spoke out of anger to friends and family, engaged in gossip- some even maliciously. (I might insert a specific example here if I remembered one).
I’m sure others know more and have better examples but I really worried about having my confession go on for hours too. I did face to face and he helped me along. He didn’t seem to mind the summary format- but I also got the impression there really wasn’t a “wrong” way to do it.
Hope it helps and good luck!