I cannot help but JUMP here, so warning: this is my opinion only and it is based on my experience. It is NOT medical advice.

It seems to me that you need to "help" your antidepressant medicine with some mood stabilisers. In my case they helped a lot with obsessional thinking (I know all about the ups and downs and how exhausting and debilitating they are). I know of a couple of anti-convulsant that are prescribed as mood stabilisers. I will give you their generic name as I am in Canada and brand names vary.

Gabapentin
Valproic acid or Valproate

There is also a very old anti-depressant medicine called trazodone or trazadone (its brand name is Desyrel here) that has worked wonders for me in terms of morbid obsessions.

Once you get the right medicine, you can start building yourself up. It's a turtle's journey but it's all worth getting to the point where you are actually happy and on even keel.

Good luck

Agnès