@Jophy
I wonder if why is a rhetorical question?
I suspect that you know best why you cut yourself. Because you are the best expert on you. Likely it helps relieve some challenging feeling.
Probably you want to stop. And possibly the trick to that is to figure out what feeling was challenging you. And then find a different way to release it.
One of the first things I learnt was "this feeling is valid, and I'm going to experience it". So, can you remember what it was you were feeling the last time you cut yourself? I found a big list of 50 feelings online, wrote them on slips of paper and put them in a pot. When I felt challenged, I'd get them out go thru them, and pick out the ones that felt most right. Often, by the time I had done that, I was feeling in a slightly better place. That might be something you can try.
But actually, just reminding myself that the feeling was valid and it was OK to feel that way, really helped. If someone has told you, or led you to believe, that it is bad or wrong to feel a certain way, then that person doesn't understand what they are doing.
Trust yourself. You are beautiful.