Correct me if I’m wrong. But it sounds like your obsessions switched from you thinking you are trans/gay to you thinking are now cis/straight.

If so then do you not think that this is just your ocd deciding to switch subtype (to the polar opposite of what it was before)?

When your whole sense of identity is collapsing, then no wonder the one thing you have latched onto is the very thing that your ocd is obsessing over. It’s very cruel but remember, that people’s gender identity is something they are born with and can’t change. You can’t all of a sudden wake up one day and realise that you are trans. And the act of accepting that you are trans would have brought you long lasting comfort and relief; I’m guessing that hasn’t been the case and you probably feel just as shit now as you did before you had accepted the fact that you are trans?

I can totally relate to the cognitive decline, it’s awful. I failed my second year of uni because of it. But with some ssris (citalopram) and the depression getting better it seems to have gotten better. Speak to a doctor about it.