My boyfriend's primary partner has made it clear she will leave if we stay together. They have been poly for a while but this is the first time he has been in love with another person (me). This is a deal-breaker for her. He plans to tell her that he wants to stay with me, which will most likely result in her leaving him. I need advice on how to support him. I did not enter this wanting him to break up with her -- that was not my goal. And I know that their relationship issues are not entirely related to me, but I am definitely the catalyst. Seeking advice on how to support him through a very hard break up while managing my own guilt about being the catalyst along with my own insecurities as he grieves this relationship. No matter what, I will be with him through anything, but I am worried that if I don't play this correctly that he will grow to resent me as the cause for making a major life decision. We have only been dating a short time while they have been partners for a very long time. Thanks for any advice!