I'm sorry, but you don't. If you truly loved that person, you will never fully let them go. It's easy for me to say this, but so not easy for me to do, but if they don't love you then you can't change that. You can hope that they will someday, but you will be missing out on your life by hoping that someday you will have another chance with them. All you can do is be glad that you felt that love for at least some amount of time and hope that you can find happiness with someone else, just as good or even better. By the way, I know that it is easy for me to say all this, but believe me, I am in this same situation and it is the hardest thing you will ever have to do is letting go of someone you truly love knowing they don't love you anymore.