For me I try to avoid watching things that I know will trigger me if I'm in an already vulnerable state. I have depression and struggle with self harm and have also attempted suicide before. Watching things with content similar to that triggers me and will make me more likely to act on any urges I might be having.

Anything that would affect your mood in a negative way is probably something worth avoiding. I guess that is gonna be different for anyone, and is a case of trial and error.