When it comes to investigating negativity, trust yourself. Use whatever investigations draw you in. There are lots of ways of looking at the mind to investigate the contents, but if you look for a source of negativity, you won’t find it.
It seems to be necessary to include the practice of being with your negativity rather than trying to get rid of it. Sit down and practice just observing negativity arise without doing anything with it. Do a lot of it. This is the start of compassion.