To be able to forget bad memories you need to avoid revisiting those bad memories that are stored in your brain.
While you're avoiding revisiting those bad memories you should get out and create new memories so that you push those bad memories to the back of the brain.
When newer better and happier memories and moments enter your brain those bad memories get pushed further back.
So it is harder to revisit those bad memories.
This has worked for me and I've forgotten several bad memories and have not revisited them in my brain for years now.
I create new lasting better and happier memories and think good thoughts about things.
There's also a new drug or medicine out there that is supposed to help erase bad memories.