Shocked and heartbroken but not sure if it's justified. Thoughts?

So, this one is long-winded, sorry.

I started going out with Katie when I was 18. She was only 15, and we both spoke to our parents about it first, as we knew that there would be raised eyebrows due to the age difference. But we were so fond of each other, we were best friends for about 4 months before we realized that we were in love. We were very young, she was a virgin and neither of us had really had a serious relationship before. We didn't have under-age sex, we took everything very slow. I've never had a stronger bond of trust with anyone in my life than I felt I did with her. She was my best friend, my lover, my first love. All that lovey-dovey stuff (I imagine most people found us rather nauseating!). Anyway, you get the picture. After around a year and a half together, I went to university. It was not that far from the town we lived in, but it made things harder because we were further apart. After two gap years, the majority of which I'd been with Katie, I was at uni. I was making new friends, and getting lots of attention from girls. I am proud to say that I was utterly faithful to Katie. It wasn't hard, I loved her. I had eyes for no-one else. There was one girl on my coursethat I developed feelings for, but I never spoke of them. Never told that girl. Because I didn't want to lose Katie, and I was strong enough to accept that this other girl was my friend and it would be selfish to ruin that, go behind Katie's back. If I had chosen to go out with her instead of Katie, it would've been easier on a superficial level. I would've seen her a lot more, had more attention and regular sex. But I didn't want that. I felt there was too much between Katie and I to waste. I'd had girlfriends before, nothing serious, but enough to feel sad and a bit broken hearted about. Katie was special to me. We shared EVERYTHING. I had (and still have) lots of friends, and many of them became Katie's friends (she was a bit of a loner at school).

Last year, Katie went to university. At this point we'd been together 3 and a half years. We were so happy, we'd just taken a holiday together. Nothing fancy but it was paradise for us. It wasn't that far away from where she would go to uni. 3 months after that holiday she had been at uni for3 months and she was making a lot of noise about how unhappy she was, how difficult it was now that SHE was at uni, and how hard it was never seeing me. I went to see her 4 times in total (despite being in my third year at uniand ridiculously busy), she did not come to see me once. I met her flatmates, who were lovely. 3 girls and a boy, Luke. I didn't really like Luke all that much, he was rude and vulgar and irritating. But Katie clearly liked him. She always spoke about how he was her best friend, how she could tell him anything. And you know what? I was happy about it. I was chuffed that she'd gone there and made friends so easily, I'm a huge advocate of girls having loads of close male friends whilst they're in a relationship. It's healthy. And the majority of my friends are girlsanyway.

Whilst I was down there, I made friends with her flatmates. But one girl in particular, Laura became a good friend after I looked after her when she was drunk and nearly got beaten up by another girl. Since then we'd remained firm friends and even spoke regularly. Christmas came and went, and I knew something was wrong. I got the impression that Katie wanted to break up with me. She was hinting at it heavily, by saying things like "this is too hard" and "you never text me back, you never call me". Admittedly, I had been slipping with communication. It's no excuse but, I was swamped with third year madness. After years of supporting Katie through exam periods when Ibarely saw her, I thought she would understand. But she didn't. Over Christmas, I spoke to Laura about organizing a valentine's day surprise for Katie (our 4 year anniversary). It never happened. After the millionth sad text from Katie I thought enough was enough. She wanted to focus on her degree. I needed to focus on mine. So I went down to see her and instigated our break up. We agreed it was mutual, that we still loved and respected each other, and that we would leave the door open. She just accepted it, she didn't fight for me. I told her I didn't want to break up, but I felt it was the wiser thing to do, as it had become a burden, a weight for both of us. And I didn't want to be the reason she was unhappy. So we ended it.

We spoke quite regularly after that, funnily enough. We even met up as friends over Easter when we were both back home. It was not awkward, but it was difficult for us not to act like a couple. But after 2 or 3 weeks she told me to stop talking to her. So that she could start seeing me as a friend, which I understood. We knew that we weren't going out anymore. I knew that she could see who she wanted, sleep with who she wanted, and it was none of my business. It hurt, because I was still so in love with her. But I accepted that it was a decision I had made. I ended it, now I was living with it.

The months went by quickly. Third year was hard work, stressful and depressing. But I got through it with a 2:1, which I'm pretty proud of. I'd tried getting in touch with Katie a few times, but there was nothing. No response. I got a text on my birthday, but nothing else. In my head I thought we were going to get back together. I realize now that was foolish. But I'd been thinking that for months and she'd not let me back in. Looking back, I felt like she'd been pushing me away since she started university, but I didn't know why.

Eventually, at the start of August I called her home phone. Her Mum answered and I asked her if Katie was available. She said "No, she is upstairs with her boyfriend, Luke. They're going away for a week to Amsterdam".

I threw up. My knees buckled. I ended the call. I was in shock. Suddenly, everything made sense and yet so much was unexplained. When I called her, I couldn't form sentences, I was crying, choked up. She angrily told me she didn't need my permission when I asked her why she hadn't told me. When I asked her how long it had been going on, she paused for a moment then said two months. When I asked her if we broke up because she wanted to be with Luke, she said no. Then she said "I understand you need time to accept this".I told her I deserved an explanation, and that I wanted to talk to her once she was back from Amsterdam.I said as long as she was with him, I didn't want to know her.I ended the call.Then she was gone, off to Amsterdam. I didn't need time, I needed an explanation. One she has still denied me.

I contacted Laura and told her I'd just found out. I asked her to tell me everything she knew, because Katie had not. She was very kind and supportive, and has been a rock for me in the weeks since. She told me thatshe didn't think t hey'd slept together, but Katie had told Luke she had feelings for him in December, when we were still together. They got together before easter, and announcedthey were together after easter. She had already slept with him before we met up again in easter. just over 6 weeks since we'd broken/ So she'd lied about them being together for only two months.I felt betrayed, manipulated and utterly let down by someone I'd invested everything into. When she got back form her holidayw ithLuke (that was a horrific week)I'd hadsome timeto process everything and I felt pretty angry. I told her that she should meet up with me, and explain why she'd lied to me. She said no. I haven't heard from her since.

Now I know I broke up with her. But I feel like she manipulated me into doing so so that she could be with him, because it was easier for her to get attention and sex and support from the dude across her corridor than someone across the country. The fact that I am so easily replaceable to her is heart breaking, and whilst I don't think I'm in love with her anymore, and I don't like the person she has become, I care about her very much and the thought of them together is unbearable. I am depressed, in councelling and still in shock to be honest. That lastphone call with her was 6 weeks ago.

Am I over reacting? Or has she not behaved very well?

Even though I know I must move on and I am grieving for something that isin the past, thisis killing me. I don't know what to do. She means so much to me, I don't understand how she could treat me like this.

Thoughts?