It sounds to me that you're another individual who's hurting and feels that putting the blame on SOMETHING will make things better. You're not at fault here, nor is your ex, nor is polyamory. You both fell madly in love, made some SERIOUS decisions in the midst of all that emotionality (never good) in a quick manner, and now you're experiencing the negative consequences of that sequence of events.
I'm sorry if I sound insensitive, I wish you the best, but I want you to understand that you're grieving. We don't think or feel the best during these times in life, and I just want to show you the possibility that maybe there's no blame that needs to take place, just grief. When the pain subsides in a few weeks or months, maybe you can look back and see all the little pot holes on the way, and use that as a way to look a head and put those new understanding to good use with future loves.
Have some tea, cry it out, then do something that makes you feel a little better. Best of luck!