What do you do when an unwanted thought about your past keeps coming to your head?

There's a Buddhist parable in which Buddha sends a disciple for water from a nearby spring-fed pond but the disciple finds the water has been disturbed by a cart drawn by oxen and the water is milky with mud. He returns to Buddha and explains the water is not fit to drink. Twice more Buddha sends him for water and on the third trip the disciple finds the water has calmed and is now clear and drinkable.

Same with unpleasant memories. You may have to see the mud a few times but eventually the waters will calm and all will be clear.