How can I keep from being jealous of her?

You have to let it go. You cannot control everything.

Use this flowchart:

Can you do anything about it? If yes: do something about it. Then stop worrying. If no: don't worry about it, stop worrying about it.

Oh my... I have been "that girl." Have I ever, lol... The truth of the matter is that if you love this guy, that love it isn't going to simply disappear. And that skank hoe... I mean, w- w- wo- wom- wom- woma-, no, she's a skank hoe at best... that he's with isn't going to disappear either. You can go find another guy, flaunt him in front of the new couple and put on a couldn't-be-happier facade. And he might buy it. But if he has moved on and is so happy, to be honest, he's not even going to care. (Which sucks, damn it all to hell!)
So here's what I've learned - you just grieve the loss. You don't front. Don't be fake. You love him, you lost him, and he's happy with some other chick and you wish it was you instead. So acknowledge it and emote appropriately. And it's okay to cry, be angry, or hurt because you're a real woman, with real feelings. If you see them in passing, don't force a fake smile. He's moved on, so he doesn't care and faking being okay only aids in the bottling of emotion, which will only end in explosion. If you don't grieve the loss and allow yourself to heal, you'll only become bitter. I've spent the last 7, almost 8 years being bitter. Don't do that. It makes you a sucky, hateful person (I seriously suck to be around sometimes).
Once you've had a good cry, stuffed your face with some chocolate, indulged in retail therapy, guzzled some wine - or however else you feel necessary to cope, simply say, "Ok. He is not coming back. I deserve someone who makes me happy (clearly this guy is killing you inside) and who I make happy. I'm done now, I'm letting it go, and I'm moving on." And then you just do. There's no magic solution - it just takes time.
As far as the girl he's with - you don't hate her. You hate only that she's with him. Realize that, and she holds no power over you. He's the one you're angry with for choosing this other girl. Don't misplace your anger on her. She just fell for him the same way you did. Who knows? Maybe it'll last, or maybe he'll drop her and make some other unsuspecting soul "happy." If anything, feel sorry for her. Because she very well could be in your shoes in time. Hopefully not, because that pain is fierce, but you never know.
Ok, all of those rantings may have been completely unhelpful and useless. If anything, know that you're a great gal, worthy to be loved and doted over. There is nothing wrong with you - you're a real woman with raw emotion. You're not fake. And you will move on and find love that reciprocates. Best wishes, Love!

I mean your jealousy obviously shows that you have emotions or feeling over him. It is not that easy to tell your hear to stop loving. I ave been in your situation. I just tried to avoid seeing them. I don't know if you stayed with your ex as friends of you don't talk anymore. If you don't talk anymore that it is easier, because you can just avoid them both. Just delete him on facebook or similar websites and stop checking how they are doing, that won't be for good. That will only make you jealous or even hurt you. What I did, was that I spent more time doing what I like. Poetry, music, sport, doesn't matter what, just preoccupy yourself with things you like. And what knows maybe you will fall in love again. I am not telling you that you should fall in love to forget him, but if you do, that jealousy obviously will disappear. If no, then, well you just have to give it some time, just avoid seeing her or him, cause it will bring your memories and that is for no good in this situation. And if you caught yourself checking his accounts or whatever, then tell yourself to stop. That is what I think.:) Good luck;)