<human>: Background story:
So me and this girl dated, lets call her X, for 3 years throughout highschool and into my first 2 years of college. She went to college the year after I did and put stress on the relationship and all that kinda stuff. Anyway we get into a lot of arguments, took a break and she ended up breaking up with me after I told her we should try to work through it. I found out through my best friend, lets call him A, that X cheated on me with this guy who i already had problems with because they were too close in a touchy feely way. Was rough for about 2 weeks and I leaned on my friends to get me through it. I ended up meeting another girl and have been dating her for about a year. 

Current:

So a mutual friend between A and I, will now be known as J, called me at midnight last night. It is his birthday so I figured he wanted to go out, but I had work the next morning so I played like i was sleeping. He called two more times and texted me and he told me it was important. So I answered and he told me to meet him in front of my house. So i did and he was telling me how him, A and a few other friends, including one visiting from Texas who we will call T were at a bar hanging out. He then proceeds to show me a picture of A with his face all cut up and im like wtf happened. J then told me that T punched A in the face. I thought that A said something to T that offeneded him and he punched him in drunken rage but it was actually because it came out in conversation that A was hooking up with X a little after a month that we broke up. J knew this as well and they both kept it secret this whole time. 

Now I personally feel betrayed and hurt by it all because I treat these guys like family, and they both literally stabbed me in the back with that.

I just want some advice on what should I do from here and did he actually fuck up by hooking up with my ex gf of 3 years?

Can you condense the text into three sentences or less?
<bot>: My best friend hooked up my ex gf of 3 years about a month after she broke up with me, advice?