How do you move on from certain things from the past?

I have certain things from my past that I still hold onto. For example such as hardships I have gone through, the way certain people have treated me and how much it hurt me or still hurts me. There are times where I think about these things and I get really sad from it, but I don't want to and I want to just move on from it and let it stay in the past so I can be happy. How do you move on from the past? What has helped you?