Pay attention to your thoughts.

If there is an overall theme to thoughts (like "fuck everyone" or "I suck") then there is likely an underlying emotionally charged event feeding them all. Deep introspection can expose and help release that event, causing the "related" thoughts to become weak and not arise nearly as often.

Grabbing or pushing away thoughts are habits, and both actions reinforce those thoughts to arise again. Often people will grab at thoughts that bring them pleasure and push away thoughts that bring them discomfort, then check out into fantasizing when all seems neutral.

The more one indulges in anger, the more anger one craves and the more likely one will habitually jump into anger without forethought. The more one rejects certain life realities, the more one will dwell in fantasy and become at odds with reality, needing fantasy (or junk food or drugs) to cope with consequences of their disconnect.

Note there is a ton of nuance: every person has different triggers and hangups, so I'm just speaking generally here.

I'm not sure you can do that. You can't change your entire thought process but you can think new thoughts and focus on them. If you focus on different thoughts, you almost become a different person, gradually. I think that's what's implied here.

Great question! There’s a great book titled, I May Be Wrong, written by a Buddhist forest monk. After spending a number of years in the forest, he was asked when he got home what the greatest lesson he learned after all that time in the forest as a monk. Essentially, he said something like he learned not to always trust his thoughts. I found that very telling. Our thoughts are the result of years of the cake being baked—our past traumas, failures, losses, and even positive experiences; a number of things. But the idea is that when we have a thought we should pause and take a minute. Really think about whether the thought reflects our essence—our authentic self. If so, then we should go with that thought. If not, then we should reevaluate. All this, of course, is related to consciousness and other concepts around self-awareness.