This is like asking the meaning of life. In order to answer this question, I'd have to know how long it's been. If it's been a month, then the answer is time. If the answer is ten years, then you need to see a therapist. Mostly the answer is time. It's also about getting out there and finding somebody else. You'll forget once you're in a new relationship, but if you don't let yourself do that, then you're going to focus on the thing you don't have. Also, we tend to romanticize relationships that we're no longer in. Try to stop. Let me give you an example. I went out with this woman and she broke it off without explanation. She got married to a guy she started seeing after me. I watched her Facebook page and actually liked the guy, but still pined for her. Well, ten years into their marriage, they got divorced. You know what the guy wrote? "The last six years of my life have been a living hell." Never forget that you may have actually dodged a bullet.