[didier19]

So, I don't really know where to start.
I met a truly amazing girl when I was just a teenager. Of course I was very young, but we started dating 2 years after we met, and ended up being together for 3 years. It was a very deep and intense relationship for people of our age, but we really did love each other very deeply, and it was something that not many people had the luck of appreciating. We had A LOT of ups and downs, but when push came to shove we truly loved each other and always worked things out, as cliché as that sounds. She was kind, caring, compassionate, gorgeous, honest and everything in between, but most of all she genuinely adored me. You can judge me all you like, but I have so many regrets about the way which our relationship panned out. I cared about her immensely and always showed her that (especially in a romantic sense), but at times I was inconsiderate of her feelings and caused her a lot of hurt (especially towards the end). As always, we stayed together because the good outweighed the bad: when we were sad, we were very sad, but we were so unbelievably happy when things were good that it didn't matter.
Anyway, about a month ago we split up after I hurt her for the last time. I was under no illusions over why it was that we broke up and I know why it had to happen for her, and by the end I didn't think for a second that I deserved her at all. She said when we broke up that she still wanted us to be in contact and spoke a lot about 'in the future', but said she just couldn't be with me right now. Now though, all I can do is think about her literally non-stop, especially since I found out that she has started seeing another guy already.
What we had was so unbelievably special that all I can do is think that I'm never possibly going to find anything better out there. She was perfect in every conceivable way. All I want is to have her back in my life so that I can show her I'm the person who can make her happy, but she just doesn't want to be with me at the moment. Im finding it so hard to cope knowing that I've let her down so badly and thrown away what we had that was simply so amazing. There is no other way to describe her than perfect, she was everything you could possibly want in somebody else, and I simply do not know what to do with myself now that she's gone. She checked out of the relationship and I feel like I'm in it all by myself, I do not want to be without her but she has found somebody else and is simply replacing me. I know she said she was open to the idea of something in the future but with each passing day I feel like that idea is getting further and further away and I'm literally devastated. I don't want to move on because I simply cannot get her out of my head.

[Alty]

I don't want to move on because I simply cannot get her out of my head.

This part of your post sums it up pretty nicely. You don't want. You! It's not about you, it's also about her, and she doesn't want you. You have no choice but to move on. It's not about what you want.

she checked out of the relationship and I feel like I'm in it all by myself,

Of course she checked out. She broke up with you, which means there is not relationship. You are in it all by yourself, because you won't accept that it's over.

I know she said she was open to the idea of something in the future but with each passing day I feel like that idea is getting further and further away and I'm literally devastated.

When will girls learn that saying "let's be friends", or telling someone that maybe in the future they'll get back together, doesn't help.

Girls say this to spare your feelings. It's a lot kinder, in our minds, to offer friendship and hope, than it is to be honest and say "I don't want to be with you, it's over, I'm moving on and you should do the same". Women are nurturers, a white lie in our minds is better than telling the person the truth.

In other words, no, she isn't looking to get back with you in the future. She's saying that to spare your feelings, to give you hope. Secretly she's hoping that in a few months you'll find someone else and forget about her, and what she said about the two of you getting together again in the future.

I do not want to be without her but she has found somebody else and is simply replacing me

She didn't replace you, she found someone that she feels is better suited for her. Will this new relationship last? It's possible. But you're not replaced. What the two of you had, wasn't working for her. She chose to move on because she didn't see it lasting for the long haul.

As for you not wanting to be without her. Well, you have no choice. Like I said, it's not about what you want. You can't make someone want to be with you.

You need to accept that this is over. I know it hurts. Breakups are never easy, especially when the other person finds someone else. But, you have to let go of this dream that you'll be together again. Don't spend your life waiting for her to come back. Go out, have fun, get a hobby, and go no contact with her completely. Delete her from your Facebook, and any other sites, delete her number, don't text her, don't call her, don't email her. Go no contact and work on getting your life together. That's the only way to move on, and that's exactly what you have to do. You have no choice.

Good luck.