My best friend (21 M) slept with my Ex (21 F) and I (21M) don't know how I feel.

As the title says my best friend slept with my ex. I meet my ex early during our freshman year of college and we immediately kicked it off. That year was probably the best time of my life. It was the first time I had someone to talk to about anything and I felt truly happy. She was my first love. But over the course of our 2 1/2 years together we grew distant and stopped feeling the same for each other. Being my first girlfriend I didnt understand that a lot of what she did to me during our relationship was pretty ****ty. She stopped trying and I ending up dragging our relationship on for an extra year. We ultimately broke up at the beginning of the summer when we were both interning in the same city. It was hard, but I knew deep down it was the right decision. I went no contact to get over her. While I tried not hear any news of her it eventually got to me that she was having a blast since breaking up and I was devastated. I was hurt by the fact she was able to get over me so quickly. I became depressed. My best friend really help me get through my break up. He gave me so much advice and told me to focus on myself. Honestly he gave me the will to get over my ex. About two months after breaking up and while I was interning, my ex went back to my home town where my best friend lived. They were still mutual friends and I respected that. Well he ended up throwing a party and she went and they ended up ****ing at the party. He apparently was really drunk and barely remembers anything. He felt guilty about it but waited a week to tell me. I was livid when he told me. I was just about over my ex and that sent me in a spiral. I ended cussing him out and then breaking NC and cussing out my ex.
Now I'm confused, angry and sad. I know I have no right to tell my ex who she can be with but I feel betrayed by both of them. I feel like I'll never be able to trust anyone ever again. Am I justified?
Tl:dr - My best friend and ex slept together and I don't know how to feel about it
Edit: Thanks guys. After thinking it through I shouldnt have broken NC and cussed out my ex. She has no responsibility towards me. However, I am cutting my friend out of my life. I'm going to focus on myself and become a better person.


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