Photographs left in a sunny window don't vanish. They go pale over years, and one day you look and the reds have gone entirely and you cannot say when.
Bad memories behave more like that than like deletion. You will keep the facts. What drains out, slowly and without much involvement from you, is the charge — the thing that made the memory arrive with your pulse already up.
Worth knowing on the days when it all seems fixed and permanent. Fading isn't a decision anyone makes. It is something that has probably already started.