My worst break-up ever

I bet you think I'm going to tell you a long sob story about some guy I was madly in love with who broke my heart and turned me into a pathetic mess. But no, it wasn't a guy, it was a girl. My very best friend ever. She and I met when we were 15 years old and became best friends. We were best friends in the way that only teenage girls can be. We spent all of our time together, discussed everything we ever thought about for hours on end. If we didn't actually see each other one day, then we would talk on the phone multiple times. Every single aspect of our lives had to be shared with all the gory details. This went on for years, all through college where we lived in an appartment together our senior year.

And then the most cliche thing ever happened, a boy came between us. I was dating this guy (although, I think date is perhaps too strong of a word) and I did not expect him to be faithful but I certainly did not expect what happened. My best friend slept with him and then neither of them bothered to tell me about it for a week. A week that he and I spent together back east. And then she calls to tell me the details and he sits there watching me have this phone conversation with this little smirk on his face knowing the whole time what she is telling me. Oh, but it gets better. He decides he really likes her better. She is upset that he didn't tell me and then we ended up in the bizzaro love triangle where he and I were both in love with her but she wouldn't give us the time of day. I contributed to this situation by being a complete bitch to her. I just couldn't help myself. I tried to make it not matter. I tried to tell myself that he was a total asshole and didn't matter at all and that she and I'd been best friends forever and we'd get over it. But the more I tried to pretend I wasn't completely distraught, the more I would say horrible snide mean things to her at every opportunity. That whole year after was easily the worst year of my life.

I've spent years wondering what I did to her that made her want to do that to me. My therapist suggested that perhaps I didn't do anything. Perhaps it really had nothing to do with me. But still to this day, I feel like I somehow deserved it.