You can't. Unless you got hit on by a car and get amnesia
Let it go.
By being honest with yourself. It's very hard to do, but it's a measure of your maturity of soul.
forgetting isn't what you need to do though, it's forgive them and yourself, let go and move on - easy to say though
Find someone new you can't live without.
Beer. And then more beer.
Tell me please when you figure out-_-