I'm sorry this has been so hard for you. I think you're struggling so much because your T ending things has activated some of your intensely painful core beliefs about yourself and about other people. So it's like dealing with a huge loss of somebody you care about while simultaneously splitting open a bunch of horrendously painful thoughts and feelings that you carry with you because of your past, your family relationships, etc. I have had the same thing happen on a smaller scale when my T goes on vacation and it triggers a shitstorm of negativity for me. It's like my coping skills fly out the window exactly when I need them the most.

I don't know how to deal with core beliefs except through therapy, but maybe there are other ways to access and work on them. I don't want to suggest therapy because you have decided that it's not for you. But you have also admitted that the time you spent with your T was not really therapy since things were more friend-like, so maybe you haven't actually really given therapy a try? Just a thought. However you decide to move forward, I hope you can find some peace and closure for yourself.