What is the most painful betrayal you have experienced?

About 4 years ago my best friend and bf (now ex) were sneaking around behind my back. They also slept with each other multiple times so I was glad I hadn't slept with him while I was with him. Anyways when I found out I just stopped talking to both of them but afterwards she became hostile towards me for no reason even though she was in the wrong. Like basically showing him off as if she won a trophy and tried to rub it in my face. My ex however didn't seem to regret it until like a awhile later though but long story cut short she got knocked up and is now stuck with a toddler while I'm free and have my life back. My ex occasionally tries to pm me but I haven't responded to him for a long time I think he gave up. I guess things happen for a reason.