Hello sry, there's a thought exercise that goes along the lines of 'don't think of the white bear' - what happens is that you of course immediately think of the white bear. We can't make ourselves forget things, our brain seems to like to decide by itself what it will let us forget - usually things like where we have left keys!
I know what you mean by having a repetitive upsetting thought or memory just spring back into your head. Sometimes it can be a song that triggers it, or seeing a picture of a person that reminds of you of someone. The best we can do is acknowledge the thought, acknowledge the flush of sadness that comes with it, and accept that it is ok to have this thought and this feeling. It won't make the unpleasantness vanish, but it will evaporate much quicker.
