Don't give in to the urge. whatever you're feeling, it will pass. Trust me. Plus wouldn't you feel better after knowing that you did not cut yourself? Something to congratulate yourself on..
Something I used to do when I struggled with SI -- get a red marker and draw red marks on my arms and legs. It helped me tremendously to get the negative emotions out and the best part was I did not have to cut myself which meant I did not have to try to conceal it from friends & family & the general public.
Best of luck