I, too, would agree with what bookbug said. But, please, in your grief, don't allow yourself to fall victim to blaming a thing, rather than the person. It's easy to think that if you hadn't "let" him be poly, you'd still be together. And maybe you would. But you couldn't prevent him from meeting and falling in love with this other woman...at best, he'd end up cheating (and lying to you), or go off to have a secret life with her...at worst, you'd end up in the same place you are now. Clearly he was never committed to you....preventing him from "committing" to someone else (do you really think she won't end up getting betrayed like you did?) won't solve anything. He wasn't right for you. If he was, poly would only bring you closer together, not tear you apart. He may just not be a good person....
If I can give you just one piece of advice to help move forward, it would be this:
Focus on the person, not the relationship style. If poly truly doesn't work for you, then that's okay. Find someone who is monogamous, who craves that sense of belonging and possession that you do...that you are HIS only, as he is YOURS only....that is a beautiful thing to many people.
But don't for a second think that you can find a person, tell him monogamy is required, and think that will prevent your situation from happening again. You'll only get hurt, sweetie.