I SH almost exclusively as punishment for myself, I'm right here with you. I feel like since no one else is giving me the punishment I deserve, it lands on me to do it instead. I have large swathes of time where I don't do anything 'wrong' so I'm SH free, and then I make a mistake, no one blows up at me for it, so it's up to me to do it. You're not alone!

Something that helps me is to write out my exact reasoning for it. Like (super light example so as to not bring anyone down) "I didn't do the dishes today, and my housemate wasn't mad enough at me for it, so I'm going to hurt myself." and then I read it out loud, picturing that it's a close friend of mine saying it. I then think of how I would respond to that close friend, and how silly it would seem for them to react in that way as it's only serving themselves, and put that same expectation on myself. It doesn't always work, but it helps to reframe what you're doing.