How to forget your lover?

Its very hard and what works well for one works less well for another. A fundamental is not to focus on your pain and lack of him. An aching heart is like an aching tooth, it demands attention. Focusing on it only makes the pain worse. So as soon as you find yourself doing that, force yourself to think of something else instead.

Do lots of things that focus your attention on them, hanging out with close friends, playing a fast sport, and such, and avoid, as best you can, situations and things that will bring back memories of your times together, such as the music you used to listen to and the places you used to go to. Time is the only healer, but you can help it along with the sort of thing I suggested.