Idk if you mean cut yourself with CTB purposes or only for SH, but anyways i'd say that for me, it works just doing it once, then repeat the same, each time deeper until I get what I want.
Personally, thinking about all the bad things that have pushed me 'til that point don't work. It's like thinking about the reasons make me feel sadder but also make me feel more tired, so I end giving up the SH trial.
If you search the motivation to do it, just analyse the things that push you to the edge. For me the external things that make me feel bad, don't affect me so seriously as thinking about how awful am I. Thinking of all the things that I did, the bad memories that I created to myself is what truly motivates me to SH.
I don't really think I helped you, but anyways I hope you find what you're searching for.