I don't normally post on this part of the board, but I'm posting here because for some reason this one struck a chord with me. The exact same thing happened to me when I was 16. I'd been with this girl several years - we grew up together - and I thought we were going to get married etc. Young though we were, I really loved her, and I never thought it would end. Then, without warning, she dumped me and took up with someone else.
I was devastated. The pain was worse than physical pain (literally). All I could ask was why, why? What did this other guy have that I didn't have?
It was a dark time in my young life.......but it passed.
Now, many years later, I look back and I realise that there was nothing wrong with me. The other guy didn't have something I was lacking. It's just that my old gf and I grew apart, that's all. As we grow up, we develop, mature, change. Sometimes that means we grow apart from people we were once close to. It's sad, it hurts, but it's life.
Nothing I or anybody else can say will make the pain go away. You have to ride out the storm. But it WILL go away...in time.
You might feel now like you will never love anybody else like you love her. But you're wrong.