hey im going through the same exact thing. i was clean for about 2 years and then one particularly bad night it just all came crashing down. with self harm after you do it long enough, it becomes ingrained as a coping mechanism. you're right that its like an addiction. your mind and body never forget what its like even long after you stop. in situations where you're under stress and have no other outlet, you'll do anything to get a release.
ive never experienced the rush or high that some people talk about, at least not that im aware of, but even still, i find myself addicted to it. i think about it every single day. im honestly really disgusted by it and i hate the immediate aftermath but its like im driven by some unconscious primal need to do it again.
i've yet to find a reliable way to resist it, especially if you don't really have any desire to resist it. the best thing to do is just to postpone it as much as you can, like say oh ill do it in 10 minutes, okay ill wait another 10 minutes, okay another 20 minutes... sometimes that's enough to curb the need. also filling up your schedule with things that take place during the times you usually sh, that has actually been the only thing that has really *forced* me to stop. none of these things really address the core issue though. and they won't work if you don't want to stop. so i guess this isn't the best advice haha... but i tried.
im sorry youre going through this. even though im currently in a place where it feels like its only getting worse, i have some hope itll get easier. take care.