Well, holy shit. I am so sorry. That's awful.
I truly do know how you feel. In the summer of 2010, during the week of my 29th birthday, my boyfriend/best friend ended our 4-year (open) relationship in a way that was a total shock to me. He blamed me for his impotence, said I had never been his "real" girlfriend, that we'd never had emotional intimacy, and that I must have no friends if I thought of him as my best friend.
When I called my occasional/casual lover (also of 4 years) to commiserate, he informed me that he had found "real" love and was getting married (and, by implication, that he would never see me again, not even to say good bye).
When I went on a family vacation the next week to recover from this double loss, my brother was a complete asshole to me [for reasons totally unconnected with my dating relationships] and I stopped speaking to him for five months.
When I called my third (and only other) friend-and-lover and begged him to let me cry on his shoulder, he told me that he had been "merely tolerating" me in his life and that there must be something wrong with me if I was blindsided by the departure of the other two men.
Strangely, I had met and/or gotten together with all three guys in the same month four years earlier, so it was darkly poetic that I would lose them in all the same month too. At the time, I think I felt that I had been greedy or selfish to maintain involvement with three men; so surely I deserved to lose them all.
Meanwhile, the mutual friends I shared with my ex-boyfriend blatantly cut me out of their lives, which also meant cutting me out of the professional life I had been establishing for 5 years. My dearest platonic friend never spoke to me again. Another friend of my ex told me that I had been my ex's "practice girlfriend" and that I should have known it wasn't a "real" relationship.
In the space of a few weeks, I had lost the 5 people closest to me (my three lovers, my brother, and my best female friend) all in one blow, for no reasons that I could understand. So yes, I know how it feels.
I went completely crazy for 6 months. I sobbed for hours every day. I became physically ill. I could barely get out of bed, but I couldn't sleep either. I developed obsessive thought patterns. I tried medications with bizarre side effects, and a therapist who could not contain her shock that I had been involved with MORE THAN ONE man. She recommended a $2000 psychological evaluation.
What I really needed was to grieve. And to wallow in self-pity (it's okay to do that if you need to). And to forgive myself, because I damn sure thought it was all my fault and that I was a complete failure.
Honestly, it's the three-year-anniversary of this series of events, and only recently have I begun to feel truly better. I had to rebuild my sense of self piece by piece. But I've done it, with time and a lot of self care, and by sticking to my conviction that I truly want a life that involves dating relationships with more than one person.
The self-doubt about non-monogamy was the hardest to get over. Logically, I should have given it all up--the three "wonderful" men in my life all dumped me spectacularly. My relationships weren't "real." And yet, that was the one thing I knew I truly wanted, to be free to have relationships with multiple men.
Don't give up, Cleo. But do take the time to grieve without beating yourself up for it.
Single, straight, female, solo, non-monogamous.