I [m/21] need to move on from her [f/20] after a five year relationship.

I just got out of a longterm relationship and am having a lot of trouble moving on. I know I need to, but it's been hard just keeping myself from trying to get back in contact with her, much less forget about her.

As a bit of background, she and I met in highschool, her freshman year and my sophomore year. We started dating fairly soon after meeting, and dated throughout the entirety of high school and into college. She moved for college, going to her dream school that is four hours away, while I stayed in our hometown. We made the long-distance thing work for a while, and it went decently for a year or so, though obviously we missed each other terribly.

There was a mutual friend of ours that was a big part of our relationship. I've known the guy since elementary school, and since the two of them got along well and he and I were great friends, the three of us would spend a lot of time together.

The two of them started spending a lot of time together when I wasn't available about eighteen months ago. I was fine with this, since I trusted both of them and since I knew that she didn't have many friends in our hometown, so it was good for her to have someone other than me to hang out with. Besides, the guy she dated before me was extremely jealous and controlling, to the point of accusing her of cheating for sitting next to a male friend at lunch. I've had issues with jealousy in the past, and didn't want to put her through that sort of thing again, so even if it sometimes bothered me that they were spending so much time together, I told myself it was fine.

Fast forward to this April. I'm woken up to several text messages at six AM on a saturday morning, from a mutual friend of myself (let's call the mutual friend C) and the aforementioned friend from elementary school (to avoid confusion, now called T). C had received a phonecall from T around midnight the night before, asking for a ride to the Greyhound Bus Station in the nearest large city by 3am that morning. T was apparently acting strange, and hung up when C asked him if he was going to visit my girlfriend.

C asked him this, and was worried by the way he was acting, because T had talked to him, numerous times, about his romantic feelings for my girlfriend. This bothered me, not only because T had let himself develop feelings for her and continued to cultivate them, but because in the messages C forwarded me, T talked about the three of us by comparing us to characters from Don Quixote, in which the character I was being compared to asked his best friend, T's analog, to test his wife's faithfulness, and then the best friend and the wife fall in love and run away together.

I think being pissed off about this was understandable, so I tried to get in contact with T and with my girlfriend, and got no response from either of them until several hours later. I talked with my girlfriend on the phone, and she seemed surprised and uncomfortable by what I was telling her, and said she would handle things on her end. T then contacted me, and seemed legitimately regretful, though in hindsight I feel that he was only regretful for being caught. I told him I would give him one chance to talk to me about it before I made any snap judgements, and though we weren't able to talk for several days, we did eventually sit down and talk.

In the days before the talk, my girlfriend was adamant that I not tell her whether or not she could continue being friends with T, and I agreed with her. I didn't think it was my place to tell her who she could and could not be friends with, because at that time I didn't suspect any wrongdoing on her part.

When T and I talked, I told him what my girlfriend and I had decided, that I would not tell her to stop being friends with him, but I told him that, if he wanted to stay friends with me, I needed him to promise me transparency. He didn't choose to have those feelings for her, in my mind, but he needed to be honest and open with me about them. If he was going to visit her, invite me along, even if he knew I couldn't come. I'd appreciate the honesty and the attempt, at least. So T and I decided to try that.

He and I didn't speak again for another two months, apart from him asking me to buy him alcohol (he was underaged and this was the first time he had contacted me since the incident, so I refused) and once at a mutual friend's place where T was drinking very heavily.

June rolls around. I help my girlfriend move out of her dorm and into her new apartment in the city her college is in, and then I don't hear much from her for the next couple of weeks. She's working while she's down there and has never been the best at responding to texts, so it doesn't worry me too much, until she calls me and tells me that we need to talk, and that she wants me to go outside and take a walk while we do, so that I don't disturb my family. When I do so, she tells me that T and herself have been in love with each other for a year at that point. A lot of the things she told me conflicted with each other, but if I assume both of them were telling the truth, then I can say this: They were never physically intimate. They did not know that their feelings were mutual until April, when I called her about T going to visit her. T visited her again, a week after I helped her move in to her apartment, and stayed in her apartment for three days. They had been spending long hours on the phone together while she was away, they would each post music or pictures to facebook with the express intent being sharing these things with the other person. They had admitted their love to each other, and T had made his promise to me with no intent on keeping it, because he wanted to pursue a relationship with her. In my mind, she had been cheating on me for the past year, emotionally at the very least, though not physically if the two of them are to be believed.

She and I talked a lot that week, because I wanted to try and make things work between the two of us. Over the next month or so, we cried a lot, decided we both needed space to figure out what we wanted, said that we wanted to go forward as friends if we couldn't make our relationship work, and decided to try being together again multiple times. We went back and forth on what we wanted for that month period, including a weekend when I went down to stay with her to see if we could figure things out. Throughout the whole ordeal, I was adamant that I wanted to work things out and that I still loved her, and she seemed to feel the same way. I also told her that I wanted to stay in her life, as a friend, even if we couldn't work things out, but that the one thing that I couldn't deal with was her choosing to get together with T, because I've cut him out of my life completely. She told me this wouldn't ever happen, because she knew how that would look.

Long story short, that's exactly what she did. I only found out because T called C to tell him about it and to suggest that T and my now ex and C and his girlfriend should go on a couples' date (To which C responded that neither he nor his girlfriend wanted either of them in their lives). I've spoken to her twice since then, once to arrange a day for me to get something that she'd borrowed from me back and for me to give back all the things that she's given me that I don't want around anymore, and once on the day we agreed to meet, where she told me that I didn't have to return anything and I took my things and left, though she kind of seemed like she wanted to talk.

In hindsight, our relationship wasn't great. I very much cared about her, and I honestly believe that she loved me for most of our relationship, but after talking to some friends who've known us through the whole relationship, I've had more than one who have told me they thought she was borderline verbally abusive. I don't think she was (because I don't think anything she did was on the level of real verbal abuse), but she was certainly quick to snap about things that made her unhappy, willing to take her anger at other things out on me, and oftentimes downright mean when she was stressed or unhappy.

She's completely out of my life at this point. I've deleted her on all forms of social media. I hate her for doing what she did to me, honestly I do. I haven't talked to her since she returned to her apartment in the city where she goes to school, and I know that I shouldn't. Doing so would just be bad for me, emotionally. Problem is, I still want to. I can't stop thinking about her. I can't stop missing the good parts of our relationship, missing the physical contact and simply being able to hear her voice. I keep getting the urge to call her, to text her, to check her social media. I know I shouldn't try to get in contact with her: both her and her new boyfriend have shown that they don't give a shit about me, or about how their actions affect others, but I can't make myself forget about her, and I'm having a hard time fighting those urges. I don't know how to move on. I've been in a relationship with her for a quarter of my life at this point, and I'm not really sure what to do now.

Sorry for the Wall of Text, guys.

tl;dr: Girlfriend dates a friend of mine behind my back for a year, tries to work things out with me while saying she would never get together with the friend, and then proceeds to get with the friend. I know I need to move on but I'm having a very hard time fighting the urges to try to get back in contact with her.