It takes time. But here are some thoughts.
1. You need to get busy doing other things so that you can distract your mind. Right now, your thoughts are creating your emotions, and if you can't stop thinking about her intensively, then you will have a harder time letting go.
2. You may want to forgive her for anything she did to you, so you don't hold onto any resentment.
3. Realize that the relationship taught you something to make you stronger in life - figure out what that is and move forward with that information
4. Remember, she is not good for you. You broke up for a reason, and most likely the girl you are longing for is the girl you first met, not the one that you now know she is.
Hope it helps a little.