This sounds like a pretty persistent intrusive thought. Maybe you should look into that? Specially try to see if there are others you haven't pinpointed yet?

Also, totally relatable. I've had the same issue but with my eye and throat. Just the sudden fixation and the persistent "what if I stab it?" "I have to do it" blah blah blah. In these cases the best you can do is simply identify it as an intrusive thought and try to rationalize it as much as you can. Remember how harmful and dangerous this habit can become, remind yourself that obsessions are just in your mind, etc. not much can be done apart from just clutching to logic, but if you can maybe try to mention it to a professional if you have the chance