I understand your feeling of guilt.
I was encouraged by my long time friend to join her marriage. We would be a triad. Her husband and I immediately liked one another, and swiftly grew to love each other. What I didn't realize - and in all honesty they were not cognizant of either - was the problems in their marriage. Probably the kindest thing I can say is that my old friend suffers from a mood disorder. She is capable of holding it together in public, but behind closed doors if she was unhappy for any reason, her husband became an emotional punching bag.
Because she was only his second serious relationship, her husband basically thought this was how women behaved. He didn't like it, but didn't know it could be any different - until I moved in. I am easy-going, rational, and even- tempered. Then he came to understand that it didn't have to be that way.
Fast forward: I move out. They see two different marriage counselor's. She still fails to see why she should refrain from taking her bad moods out on her husband. A year after I left, and they are separated and divorcing.
Do I feel guilty? Yeah, sometimes. I can honestly say if I had never come into their lives, they might still be together - at least for awhile. That said, I did not create the problem between them, nor did I interfere with their attempts to resolve it. The fact that she felt no responsibility for learning emotional restraint, and that he chose not to put up with it any longer are their own personal choices.
So while I can understand that you feel guilty, logically you know the guilt is not warranted, right? You did not cause the issue between them, nor did you interfere in their relationship. The fact that their relationship failed (despite the fact that it was your bf who made the fi al decision to end it) is due to behaviors and choices they made.