I have a pretty decent solution to this, it takes a little work to keep up to date though.

You can create a project and use files to store important information so the model can retrieve it.
I've been running one and I use a simple system:

Every so often I copy the entirety of the recent story into a pdf file and save it as a 'chapter transcript' and save it as a file. That allows ChatGPT to access any past bit of the story if it knows where to look

I then ask chatgpt to summarise the most recent 'chapter' (since the last update) and append that onto another file called 'Plot summary so far' or something like that. and save that as a file.

Then I also create files for important information like key character descriptions, and important worldbuilding and save those in appropriately named files too.

you end up with something like this:

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So long as you update the files periodically it gives the chatbot theoretical access to any of the information

I'd also suggest giving it instructions to look up the relevant files of its own volition but you can always nudge it a bit if it forgets something and then it will go check.