Thank you all for the messages and advice. I've done three things since I posted this thread.

First I'm seeing the doctor about proper antidepressants, not self-medicating on uncontrolled and huge doses of 5HTP

Second, I contacted my ex and asked her about her own therapy and about the other woman. She told me she is in therapy and the other woman is a friend (but obviously in a little more detail than that!). This is the first time she's answered anything I've asked her directly (and very promptly) so I believe her. I still love her but I am going to try my best to draw a line under our breakup. Maybe in 6 months to a year, when she (and I) are better, maybe we can salvage something of our friendship. We were the very best of friends before we had a relationship.

Third I've made arrangements to see my oldest friend for dinner and drinks. It'll the the first time I've gone out of the house on my own for about two months.

Baby steps no? Little tiny achievements each day to help me climb out of the hole I seem to have dug myself into. I will continue with my therapy. Now my therapist has opened a can of worms, I'm going to have to deal with the little blighters one by one. It will be hard but necessary for my sanity.

Thank you all so very much for taking the time to care x

PS: And as someone totally new to PC, what are all these little messages that have been appearing under posts? Ones that say "hugs" and "thanks for this".