You need to learn how to be hopeful and think positively. If you let your negative feelings get in control, you migth end up loosing him. When you get stuck in negative thoughts, think about the fact that he chose you rather than the other girl, that he loves you now.
The one who loves more in the beginning of a relationship always has to fight for his/her love. By being a considerate companion you will keep your loved one. Accept him the way he is and give him a lot of appreciation. You love the one that makes you feel good about yourself. In time the love of your partner will increas and grow stronger, maybe even stronger than yours.