Pre-pandemic I was playing at a table with some fantastically creative people. We had 4 long term campaigns running for about three years before life got in the way and we all drifted apart. I was a player in all of those. We also had a stable of characters for one-shots when someone couldn't make it, and I usually threw those together to run on the fly.
My advice for keeping them straight, first and foremost, is to make sure someone is taking notes. One of my characters was a bookworm intent on preserving the legacy of her adventures, so I kept a journal in character for that campaign. I still have it somewhere.
Critical Roll was just starting to get big, and my table was comfortable with audio recordings, so I recorded our games on my phone and upside then to a Google drive so any one of us could listen to them between sessions if we wanted. I had a very body-heavy, brain-light job at the time, so I often listened to last game during work the day of the next game--like our own personal podcast. We had a player (another player's girlfriend) who was so impossibly soft-spoken, but also so full of sass. I sat across from her and missed a lot of her sarcastic asides during game--but the recording didn't. Acknowledging those with her brought her out of her shell eventually.
Anyway, be invested. Take notes however you need, or don't be afraid to ask someone else to. And see if your table minds being recorded. I still listen to those campaigns once in a while, and I wouldn't trade those memories for anything.