When told to put something in the past, how do you make it stay (in the past)?

I was severely abused as a child. I am now 38 years old and out of the family of origin. As many of the answers on here have said to put the past in the past, I would like to know how? How do I prevent the flashbacks which come upon me during the day, or the nightmares that interrupt my sleep several times every single night? How do I make the past stay there?