Letting go doesn't mean that you don't care about someone anymore. It's just realizing that the only person you really have control over is yourself.
If you want to forget something or someone, never hate it, or never hate him/her. Everything and everyone that you hate is engraved upon your heart; if you want to let go of something, if you want to forget, you cannot hate.
The truth is, unless you let go, unless you forgive yourself, unless you forgive the situation, unless you realize that the situation is over, you cannot move forward.