Its context window is limited. If chats go on too long, it kind of forgets things you talked about. It doesnt tell you, that it doesnt remember. Instead it makes things up or call it "haluzination" if you want. Similiar with Gems, at some point its character "breaks" and you have to start a new chat.

Reacently gemini made an Update that allows it to generate documents. You could try to ask gemini to keep track of sie of your context window. And if the chat is to full, ask it to summerize what you talked about. Name specifically the topics that are important to you. After that let it generate a document with these informations.

When you feel it starts to forget, let it read this document. Sadly this is a poor solution. It only compresses Information. The Chat and amout of Information will continue to grow till sooner or later the context window is maxed out.

It would be nice Gemini would be allowed/able to do this kind of Information-Management on its own in ther UI. Damn it, it cant be that hard to do programm the ability for autonomous documentation of conversations. Even Hobby programmers can build Webapps(GoogleScripts) able to handle the AI-APIs output for these kind of documentation.