My heart breaks for you, BrotherMan. I know how you feel.
I hope there will be other, much more experienced, polyfolk come on with deeper advice. I can only share what I think will help.
If your needs have been neglected in favor of the new bf and NRE, it is completely foreseeable that you would feel rejected and pushed aside. People do crazy stuff in the throes of NRE. If you can, understand this, and forgive it.
To fix this and put things back on a good track, you need to be clearheaded and strong. And it all boils down to this: figure out what you need to be okay, to feel loved and valued. Then, ask for it. Clearly, in terms that well-intentioned crazy people can understand.