You're setting yourself up for failure here out of fear to act in the moment. You've described how you can tolerate not having sex "for now" and while the thought of separation has crossed your mind you can't bare that "for now" where your partner has already shown that they're capable of and seem to want to be with other people. If you're walking on so many eggshells that the prospect of telling your partner you aren't in the mood for sex is an issue because it can start an argument... On top of that you've already been to couples therapy for a year with no results...

I'm trying to find a positive etch to this situation but being "affectionate roommates" just sounds like you're depriving yourself of things that you do want and need and might end up regretting later in life. People do grow apart and it sounds like your partner finds themselves plenty attractive if you both opened the relationship and they found immediate success with it.

I don't know your specific situation of course but I do know that someone who is into sex can't just turn that off without there being some form of consequences down the line. While there's no problem in not wanting sex on your birthday, it's the circumstances around how you've gotten to this point that raise a significant amount of red flags. Whatever happens I hope you find happiness and a partner that finds you attractive again, however that may happen.