It sounds like a lot of the drama in your life is from your boyfriend's girlfriend.
How are you doing with your feelings of jealousy, etc? How is the relationship between you and your boyfriend if you take the other-girlfriend-drama out of the picture? How is their relationship otherwise, if you take your involvement out of the picture?
Do you have any concerns that him moving from primary with her to primary with you, and now looking for a new girlfriend, may eventually lead to primary with new girlfriend?
If their relationship is falling apart "because" of his polyamory, then there are probably communication issues. It's impossible for me to say whether they're his or hers or most likely both. If their relationships is just falling apart anyway, and she's lashing out because you're something to lash out at, then stick with it.
At the end of the day, whether or not things work out with him, I wouldn't give up on poly. If you have the capacity to love more than one person, then it's just a question of developing communication and time management skills.
It's inevitable that some of your poly relationships will fail, just like all of your past monogamous relationships failed. If every relationship worked out on the first try, we would all be married to that boy/girl from grade 8