Im not an expert or anything so you should probably listen to someone else first but....

I think the most obvious answer is to keep yourself busy, distracted, suck up more of life, do something different, introduce yourself to new things, people, break a pattern etc. As the saying goes: "An idle mind will linger in dark places".

Instead you could put up mental barriers, of course this all depends on what state your mind is in, how old and conditioned it is etc. The most basic way of achieving this would be to simply clear your mind by focusing on something else and running with it, or associating such thoughts with negative undertones, cancelling them out by immediately thinking "Silence, I’ve got better things to do than to linger on such things".

These are both really just forms of denial though, so alternatively with the assistance of a therapist you could run with the thoughts, dwell on them, see where their stemming from and leading to, accept them (this could be by disregarding them as nonsense) and move on ^^

I think the best advise would be to see a therapist about it, unless you’ve got the mental strength to deal with it yourself.