What chatbot has the longest memory, and is there any way to extend it?

I was using a chatbot to write a fun, long time fantasy thing that was helping me generate ideas for my own writing. It was fun, but gets ruined once the ai starts forgetting things. The whole point of good writing is to have a wide history to draw from, obviously.

So my question is, whats the best way to get the longest memory out of ai? I want it to remember things i said hundreds of messages ago, even if i have to pay for storage or something.

(Ive seen suggestions like ask the ai to summarize whats happened periodically, but i feel that loses subtle character development nuance. Eg. I dont want a sassy character to just go back to being sassy and cold after theres been a precedent of gradually moving past that.)

Any tips appreciated! Maybe i just need to wait a year or two for this to be possible.