My girlfriend and my best friend. (help!)

[chisoxchampions]

Alright, so last summer my ex and my best friend of 9 years got together while we were dating. I broke up with her and never talked to her again, and after their "affair" was over, I forgave my best friend and we continued being friends, with my trust for him pretty much depleted, obviously.
So now I've had this totally awesome girlfriend who I can't believe i've landed, I feel like i'm the luckiest man in the world, you know that feeling?
So a few weeks back my best friend starts going out with my girlfriend's best friend. And ever since then, whenever I see him, I get like really really really angry. Now see here, i'm known as one of the most mellow people around. I never get angry. But now i'm having this extreme anger problem with him. Like when I hang out with all of them together, I try to be mature about it you know. But it doesn't work. I end up ruining everyone's night because i'm unresponsive and just being lame. Not being fun like I usually am. And today my girlfriend took my best friend and his girlfriend to McDonald's and something so simple and stupid as that makes me like, enraged. And I can't tell why I'm feeling that. Maybe somebody could help out?

[Mish]

And I can't tell why I'm feeling that. Maybe somebody could help out?

I think it's obvious why you are feeling that. Your friend has demonstrated to you that he is quite capable of betraying you and in the back of your mind you fear that he will do it again. I think it was a bad idea to introduce him to your new gf given his history of gf stealing, if I were you I would keep distance between them, make these two worlds not interact completely. But since this is not possible anymore you pretty much have very few choices, either to suck it up and accept what happens happens, or break up with your friend and tell him you don't ever want to see him again (don't need to give a reason), or explain to your friend how you feel and tell him to put as much distance as possible from your new girlfriend, failing that you don't want to see him again. It's hard to say which choice is best, it will all depend on what your expectations for the future are and how much you value your friendship.

[Gigabitch]

I forgave my best friend ...

No, you didn't.