>>1

> is it even possible to explain without sounding utterly and completely insane?

I guess it would depend on the nature of your obsession, your therapist would be the best to judge whether it's harmless or not.

I'd consider carefully whether you feel your therapist is worthy of that kind of trust, since it's your underbelly you'll be exposing and she'll be perfectly poised to gut you like a fish.

It's been three years, how do you feel about her?

If you decide you can trust her, just tell her that you haven't told her everything and you want to but are afraid that it'll sound a bit insane. Feel out her response and you'll know if you should or not, but perhaps you should give her and yourself a chance to at least try.