How can I not hate my body? (Self harm TW, nudity mention TW)

I hate my body. My face I'm all right with-- it's feminine, yeah, but it's me. But whenever I see myself (especially unclothed) from the neck down, it's surprising and upsetting. I feel like body positive encouragement is often along the lines of "all bodies are good bodies", and "you're beautiful just the way you are". But it's not a good body. I don't like it, and I hate living in it. I self-harmed for the first time (I do not plan to do it again, and I have a caring person in my life who is easily reachable if I'm afraid I'll do something bad to myself), and among other things, it felt like I was taking revenge on my breasts, which is where I cut. I'm fifteen in a few days, and it's staring me right in the face that I have at least three more years (and that's a preeeettty optimistic estimate) of this. What can I do?