Just FYI this is a throwaway account.

So let me back it up a bit. Around 2 or so years ago, I got my first girlfriend. I really liked her and was ecstatic to be with her. It was such a new experience for someone to have feelings for me in return, and I have a lot of great memories of being with this specific girl. I look back at these memories very fondly. When she broke up with me though, I was heartbroken. It came out of nowhere and I never got a reason as to why she didn't want to be with me anymore. Fast forward to the present, and I still have feelings for her deep down. I don't know if they'll ever go away, and I think this is due to the breakup not being mutual. Anyways, my best friend recently started talking to her. For the sake of this post, let's just call this girl "Sarah". So my best friend started talking to Sarah. Big deal right? I didn't think anything of it at the time, and didn't even consider if they were having a romantic relationship. More and more, he started acting really distant. He would hang out with our mutual friends, but not even talk to me. Later on, he finally told me that he asked Sarah out, and she said yes. I honestly couldn't even believe him. The thing is, we've PROMISED each other that we wouldn't date each other's ex girlfriends. I've never even considered asking out any of his ex girlfriends, and for him to just go against his word, makes me feel really betrayed. Even if we didn't promise to do this sort of thing, I'd think it's a "Bro-Code" given, to not do this kind of shit. And I know for a fact, that the only reason he would go for Sarah, is because he is too scared to put himself out there and meet new women. I honestly feel really betrayed in this situation. I should also mention, he's thrown me and our mutual friends under the bus for girls in the past. So I don't know why I didn't see something like this coming. He knows how I felt about her, and how I feel about her today. We've been friends for much longer than both of us even knew Sarah, so it baffles me that he would just betray me like this. Am I the asshole for thinking this way? Should I just let them date and not say anything about it?