If your friends are making fun of you behind your back, they aren't your friends. But do you have any proof of that?
You've tried to talk to this girl many times. She doesn't seem to be interested anymore. She's moved on, and you should too. The truth hurts, but you shouldn't waste time and effort on her if she isn't even really listening to you, or caring.
Keep your chin up, find a friend who understands what you're going through. If your friends now really are true friends, try to patch things up so you and this girl can at least be around each other without it being awkward. If she's friends with your friends, chances are you'll be seeing a lot of her.
I hope I helped. Good luck, and cheer up! Things will get better before you know it.