The reason that it hurts so bad is because this guy was discovered to have been a fraud. He told you that he’s always be there for you, but he said that so A: you wouldn’t leave him in the first place, or B: so that he wouldn’t appear to be the douche that he turned out to be.
Now that you know what a mean person he is, you are probably feeling a little guilty for choosing someone like that in the first place, or ashamed because you didn’t realize what a loser her was. So even though you broke up with him, you are still feeling poorly about yourself and your ability to choose wisely. Stop thinking like that right now! These kinds of guys are all about the manipulation. They prey on people who are kind and giving and loving. They march right in and take what they can from people that they know will give them the benefit of the doubt. So stop doubting yourself and just keep repeating that “I dodged a bullet, I dodged a bullet!”
And like the others have stated, you must remove all of his contact info from everywhere. From your FB, e-mail address book, phone, physical address book that’s sitting next to your phone at home. If you have any stuff from him (gifts or mementos that you kept) make a plan to get some of your best pals together and have a little bonfire with all of his crap! Do not give in to the urge to sweetly mail his stuff back to him, or to hang on to his stuff, or else you will always have that attachment to him (and you will have false memories of how wonderful he was). He was never wonderful! He manipulated you into thinking that he was wonderful so that you wouldn’t call him out on his bad deeds and he could just continue doing whatever he pleased, without regard to you.
You might want to consider getting into some very short term therapy. A couple of good sessions with a good counselor can do wonders for you self esteem.
Try to figure out where this dude is likely to turn up, and avoid those places. But know that you are likely, some time down the road, to bump into him somewhere, when you least expect it. When you do see him, just say, “Hi Jeff, how are you?” with a pleasant smile. Then say, “I was just getting some snacks. I’m going to watch a Netflix movie with some friends. Nice to see you, bye.” Resist the urge to flee or hide. But also resist the urge to cry or scream or give his new girlfriend (if there is one) a dirty look. Shake her hand and say hello. But be brief and excuse yourself. Have a bunch of pre-made excuses for why you have to leave (but make them sound legitimate so it doesn’t look like you are fleeing in terror).
If you are mutual friends with other people that he knows, let them casually know that you guys are no longer together and that it would be better for all parties if you didn’t have to co-mingle, and that you’d prefer not to discuss him anymore. You can also, casually ask these people if he’s going to be at such and such a party/event/place so that you can make other plans not to be there.
And then, last but not least, do not sit at home and wallow in sorrow over this guy. He’s not worth the tears that you will cry for him. Make it a priority to get up and out and moving and doing things that you like to do with people who’s company you enjoy. Resist the urge to sit at home and cry. And resist like the devil, the urge to contemplate the “what ifs” and “if I had just done things differently”. This guy is bad news, but guess what? He’s also history.
Eventually, you can do what I do and go on Fluther and help other people to avoid douchey dudes, to see the signs of douche-baggery and head for the hills whenever you see it.
Hope you feel better soon. Good luck and get moving! : )