I can't help getting an urge to cut the sides of my mouth whenever I'm holding scissors
When did you suddenly get this urge?
I just got the urge one day.
Well, it's a bad idea to purposely give yourself injuries. You should probably see a counselor for whatever is making you want to hurt yourself.
I mean I can build up a tolerance for it.
You shouldn't even want to build up a tolerance for such things. You should just avoid hurting yourself at all.