Advice: some people can't handle the stress of a poly relationship. (Mono relationships have their own stressors, too, of course, but we're not talking about that right now.) Your girlfriend has told you very clearly that she just can't handle being with you in a poly context. It looks like she gave it a good try and just can't deal. That's totally legit.
It's really sad. But it looks like you aren't compatible.
I know it's scary to be alone again. I'm so sorry about that.