How to let go?

Just when I was about to learn to let go. Just when I thought I can move on.
Thoughts of the past came back. Birthday, police, grandma, all these became nightmares of my life again. Dissapointments, hurts and feeling of unjust came back even stronger.

Lord how to let go?
How to forgive?
I know I should, but it so hard.
Help me...