I've had a huge crush in my seventh grade, a princess in fact. But she was stolen by one guy and they were together till tenth grade. Now I am in tenth grade and on the first day of school I get to know that she is in my van as she shifted her house. I started talking to her and we got so close as good friends there is no secret that I don't know of her. All this while I controlled my love, emotions and anger but my love hid all the feelings. And it so happens when I was talking to her on a Christmas vacation(online), we were messaging and she told me that she likes me and I told her that I Love her and we dated for two days and she said that she can't do it anymore as she is not over with her her ex, so I gave her time and I promised her that I would wait for her hiding in her shadows and her last message was that she likes me a lot. Only my best friend had known about us and after a week in my absence he proposes her and she is with him now, I WANT her, I LOVE her and I seriously want her. But its been two days that they are together and I feel like years passing with only winter around. I can't tell her that I love her and want her as I see she is happy with him and I had promised that I will keep her happy even if I am not with her. I want her not to be with him and me what to do?

You told her that you love her.
She tried dating you, and didn't feel the same feelings - just likes you a lot - very different. No 'click.'
You have to ACCEPT that she isn't going to ever love you. We ALL go through variations on this. You keep going in your school and hobbies and sports and interests and regular friends, and eventually the pain does go away, even if it feels like an eternity.
You promised that you 'will keep her happy?' No, people don't keep people happy, or let them have time off, or any of the other phrases you use that show that you don't quite understand relationships. We don't own or control or allow anything - people are free to do what they choose. She chose this other guy. You need to spend some time thinking about all of that.
As you get older you will realize that we just don't get what we want most of the time. Spend more time being your own person rather than investing your heart and soul into another person. It will make you more attractive as a person, and someday you will meet someone who loves you as much as you love her.

Were you this dramatic (some might say a drama queen) when you were around her? If so that might have been a huge turnoff for her. I know it would be for me.

As the all knowledgeable Joy said she picked someone else. It happens in relationships and your job now is to move on/forward.

In the future have a little less drama, don't profess your love immediately, and get to know someone well.

You dated for 2 days, she said she couldn't do it anymore, then she went out with someone else....consider this over and move on before you cause yourself more pain.

I also agree that perhaps you were too much drama for her...of course, I have no idea how you acted around her but if what you wrote is an indication, she probably felt suffocated.

You can hold on to the hopeless romantic notion of a great love lost, or move ahead in life and look for the NEXT great love of your life.

Or right a flowery romantic book. Life is not a book, and reality makes no sense getting stuck on a fantasy you cannot share.

Have you ever heard the expression, "All's fair in love and war?" It is sadly true. Rules are broken all the time during the dating game.

You could also look at it as she's just not that into you. You may not understand why you didn't make the cut, but obviously you didn't. She has chosen another, and that's that.

Hard? Yes it is. But it is important that you just get over yourself, realize you don't have a shot of her feeling the same way about you, as you do about her (for whatever reason), and she's obviously given you a straight answer. The sooner you accept that you cannot change the game to suit your needs, the sooner you will move on and find another girl.

Good luck.

Thank you all firstly, I have understood it. I am ready to accept that I am not the one deserved for her or maybe she isn't the one for me. I shall take your advice and move on and start giving time for me, as all this while I felt that she was living in me and now I don't even want to get into anything with her and move on, and remain her friend that's all. Thank you Everyone...