You just do. The First Step is to MAKE and STAND with your DECISION to MOVE ON. That is the first and hardest step. You have to commit to it. Like everything that is worth having worthwhile, it takes courage, dedication and the will to move on and FINALLY decide to stop being sad and continue on with your life and be happy. This is never easy. But Finding True Love and Hapiness NEver is, now is it?
It is not easy to let go off someone you shared a history with, especially when the relationship lasted for some few years. But it will reach a point you will have to accept the reality of things and move on. Get into the social scene and get to know people. This will help in forgetting your past love. Always keep yourself busy and occupied so that your mind deviates from those thoughts.
You surely can move on even with that "someone". It may be little harder & slower than usual but u'll move just fine. Are u ready to fall back or leave 'someone' behind - that is what u need to decide!? Remember, there is nothing indispensable in human life... .there's nothing u can't live without....but don't also forget that fortune favors the brave.