How to stop thinking about bad moments that have happened in the past?

If you are having a past thinking comes in your mind or friends ,family or any one tells for that I have a best idea for everyone.

Stop thinking about what has became in past but instead of that think about future and present instead of thinking past ,think about good thing which will happen today or in future .why should we think about past.

We know that it is unforgettable to think about the bad thing which happen in our life. I know we'll about than then also stop thinking past bad days and thinks about good days which are coming in front of you.