My therapist says that I can keep working with her until I no longer think I need a therapist. Which is great, but it assumes I'd be able to know this all on my own. I don't think I would.

Even if I see her forever and ever, there IS no forever and ever. People retire and tell you "have a nice life", never to see you again. Or they die. So even if there is no formal "termination" for me, there will be an end.