You need to get as much distance between you and your ex as you reasonably can, and you should avoid activities or events where you're likely to come into contact with her. Resist the urge to contact her, and delete her from your contact list if the temptation becomes overwhelming. You need to start getting used to her not being in your life, as this will help your previous emotional attachment to subside.
Realize that there's no excuse for hurting somebody in a relationship, and no matter what problems you and your partner had it was not necessary for her to do something that hurt you.
True, ending a relationship can hurt, but going beyond that and inflicting pain in addition to that which comes with being dumped is uncalled for. So understand that as far as that goes, you are not in the wrong. Often when people have been hurt badly in relationships, they blame themselves and worry that they drove their partner away. Don't feel that way.
Take comfort with friends.
Your friends can be your best support network, and can be a fantastic way of taking your mind off your ex girlfriend. Often your friends will be able to give you a good sense of perspective and reassurance about the situation, which can help you in regain your self-confidence. Don't feel hesitant about relying on them more now than you did before. It's natural after a breakup, and it's likely they'll anticipate this.
Meet new people.
Not only will this take your mind off your ex, but it'll help reassure you that there's a whole world of interesting people out there. Meeting new people is also the most reliable way to increase the opportunity for future relationships, which can be an important part of moving on with your life.