I feel so sad - he used to be head over heels for me and do everything to try and get me, and he'd just be there all the time - and now it's so different. Why is this? What do I do? We go to the same school, and it's impossible for me not to see him everyday and we've been through so much together.
First off, let me say how sorry I am for the rough time you're going through. I can't say why this guy went hot and cold on you, but believe me, most of us have experienced a passionate pursuer who suddenly lost interest.
And it hurts.
You don't say how old you are, but it's especially painful when you have to run into the creep who left you flat every single day at school or at the office.
My best advice is to stop asking yourself why the guy went cold. It probably doesn't have all that much to do with you, despite what you're telling yourself. Stop asking yourself what you said wrong, did wrong, wore wrong, ate wrong.
Give yourself a break.
Can you rekindle this guy's affection? Maybe, maybe not, and then you have to seriously ask yourself if it's even worth it. For instance, when you were together, did he have any annoying habits you chose to ignore?
Remind yourself of those annoying habits every time his cute face pops into your head.
Respond to his sudden lack of interest in kind. In other words, do not --under any circumstances --approach him. Do not email him or make excuses to talk to his friends.
Avoid him at all costs. When you do see him, keep your back straight. Smile, say hello, and keep moving.
One of two things will happen:
He'll realize he's made terrible a mistake and start coming around again. In this case, continue to treat him coolly. Do not allow yourself to become vulnerable until you're sure he won't drop you like an old shoe again. Keep yourself busy with friends and school until it's clear that he understands that you're the greatest thing to ever cross his path.
The other possibility is he won't come around again.
In this case, you will get over him in time. When that happens, it will become apparent that the poor guy isn't even in your league. You will run into him in the library and discover that his ears are too small for his head. You'll wonder what the heck you were ever thinking.
You'll move on to better things, and it'll be his loss.