I'm working on something similar right now. best bet for something as open ended as therapy would be to use embeddings to compare your most recent post to past posts, and then use text snippets or a summary of similar posts as part of the prompt.

granted you won't really be able to do this with ChatGPT, as you'd need to add another layer on top to do the similarity comparison and archiving of previous posts. but making an app to do this with the gpt api should be fairly straightforward.