To start with before my ex and I dated, my best friend and I both liked him. I was honest with her, and told her I had feelings for him and that I was sorry. He asked me out, and I said yes, and my best friend got over him and we remained best friends. You also have to know that I have a very, very tight knit group of friends that my best friend and my ex boyfriend are both a part of. While me and my ex were dating, him and my best friend were very good friends as well (she talked about him like her "brother"). My ex boyfriend and I started dating a little over a year ago, and were each other's first serious relationship. We were really close, I really did love him more that anything in the world. I broke it off about two months ago because I started to feel like I felt more for him than he did for me. I would always think of him first, and I'd notice if he had had a bad day, or something like that, and I'd do anything to make him happy. But I realized that he didn't seem to notice if I was feeling down, and I felt taken for granted. Another reason I broke up with him was that it was hard to be in a relationship with him because I'm a lot more mature than him, and I felt like a carried all the weight of the relationship. I first noticed this one day at lunch with him, and felt really angry and all these feelings just came at once. After school I talked with my best friend about it, and she told me what I felt was justified and convinced me to break up with him. I broke up with him the next day, at her urging; telling him exactly how I felt, but saying that is wasn't because I didn't love him. Now I feel like I should have waited longer before deciding to end it. It was awkward with my ex for a while, and a couple weeks later he told me he was sorry and asked for a second chance. I told him that now wouldn't be a good time, and that we had our whole lives to decide if we wanted to get back together. I knew that things wouldn't change if I got back with him, but it hurt because I never stopped loving him. We started talking and became friends again, and I began to notice him flirting with my best friend a lot. Eventually I asked him if he liked her, and he said yes. I told her that he liked her and asked her if she liked him. She said she didn't know if she liked him. I asked her (more than once) if she'd say yes if he asked her out and she was all like "Probably not, I dont really think of him like that, you know? I mean i dont like him like that". I went out of town for a weekend, and when I came back my ex had asked out my friend (this was about a month after we had broken up). She wouldn't even tell me right away that they were dating, but I had a feeling it would happen and she told me yes when I asked. I didn't talk to my best friend for two days, and then I told her how I felt. I told her I felt horrible because she lied to me about how to she felt about him, and it hurt so much that she could lie to my face. She started crying and telling me she knew she messed up and she was sorry. I started talking to her again because its so hard to just ignore your best friend, even if they hurt you that much. I hoped she would break up with him because she realized how much it hurt me and that I was more important than any guy, or at least act sorry and not act like nothing had changed. Instead she went on as if nothing had happened and continued to date him. About two weeks after I first talked to her, I told her again how much she had hurt me and how I was angry she hadn't acted different. She didn't say anything back, and now were not speaking. Im also not speaking with my ex boyfriend. Both of them act like it doesn't hurt that were not talking, like they don't care at all how much they hurt me. Every time I see them at school or think about how close I used to be with both of them it hurts so much. I'm still getting over my ex, and my best friend knew that. I miss him so much sometimes, and I miss her even more. They were the most important people in the world to me, I would have done anything for them, and now they don't seem to care that I won't talk to them. I just wish things in our friend group could go back to the way they used to be, because we were all so close.I am so hurt and confused and I don't know what to do.
In summary:
I dated a guy for about a year. Things were serious.
I broke up with him.
My best friend began to date him a month later.

Eventually it will get better if you leave them alone and make new friends. Curious as to why you talked to your best friend and not your boyfriend when things got rocky, but that's a mistake to learn from, keep others out of your business, and chose friends carefully.