RP, I don't understand - your last post is a little confusing.
Why did you feel it necessary to break up with Derby, whom you've been with and loved for a long time, and Brad, whose company you seem truly enjoy? Was that a reaction to Mono embracing polyamory now? Do you feel like if you focus more on Mono, he might not "stray" and pursue anyone else? Are you strategizing to reach an outcome you would be happier with? You want him to stay monogamous to you? How did Brad and Derby take it?
I wonder if you're scrambling out of feeling a bit desperate for things to be the way they once were - and familiar. You know, nothing will ever be the same, hon. Also, why do you say you'd be alone if you have PN? It sounds like you're really upset - or did you mean to write that PN and his gf are thinking about monogamy?