For me, bad memories consist of really embarrasing or mean things I did while drunk. I always start remembering them when I sober up. A part of me wants to just drink so I don't have to think about these things...because when I drink, I simply don't care. With more sober time, I stop thinking about these things. But in the beginning, they are memories that are tough to live with.
If you are talking about something more traumatic than what I am referring to, I don't know if they will go away without getting some professional help.