Struggling with Self-Harm Relapse

So, I have been struggling with a relapse of SH. I started after someone on my dorm floor committed suicide about 2 months ago and I’ve been struggling with it ever since. Some weeks I don’t think about it and some weeks it’s all I can think about. A couple days ago I had really bad urges and they wouldn’t go away but they’ve gone away gradually. However today I was using an exacto knife in art and he said we could take them with us to continue working on the project and my minds been all over the place so when I got back to my room and saw it in my bag my impulsive decisions won and I started cutting myself with it. I didn’t even feel the need to but I did it anyway. How do I stop doing this?