I do the same thing, I SI to punish myself. I feel better after because it takes away some of the shame and guilt. I am embarassed by the scars and so regret it later, but it just feels like something I have to do to make things "right". My t thinks it is a punishment paradigm that I learned in childhood, I mean, if I did something wrong and then got hit, that somehow "paid" for the wrong.
Just some thoughts, keep safe!