How to let go?

First post, been reading for about 3 months! Sorry for any mistakes but english not primary language, writing from old Europe. Anyway, me and wife been together for 16 years, we are both 38 with 2 kids. Marriage as been pretty much trouble free for this 16 years. Both of us share values and beliefs, we push each other, and sex as always been great. I get the ocasional turn down, i do the ocasional turn down as well. Sex has never been a problem, she flirty and likes the attention, i flirt as well, or at least i did. But 3 months ago we got a stumble. She made a new friend, and got really close, didn´t bother me at first, then I notice a change in behaviour, she was emotional connecting with this guy, texting and facebook. At this point is nice to point out that we work together. i confront her, she first denied, but being true to herself admited that she let things go to far, and the guy was getting the wrong impression that he might get something out of her. I demand mesures. She took them. Cant blame her for that, she did what I expected of her, normal behaviour resumed. Problem started here, with me. I was not capable of letting go, I was not capable of shutting up about it, I was not capable of NGAF. I become BETA pussy... She told me this, in her on words: "I need my man back". That is when i found MRP. I read the posts, and the side bar, and it hit me... Without even knowing it i was able to mix up alpha and beta, but that event shatered my confidence, i become beta all the way, even stooped riding the motorcycle. I lost wheight, and gain muscle (sex was always great, but now foreplay is awesome, she likes the new muscles!!!), girls complement me all the time, but i couldn´t flirt with them. Need an hollyday, so we did, and that helped me a lot. Since then i´m slowly building up my levels of confidence, i manage to shut up. There is one thing that botheres me still. Not able to let go. I think about that event all the time, i gained a real grudge against the guy...so how do you guys let go? Cause that i cannot figure on my own.