I agree with both above me! Like Sannah said, a therapist can help a lot, if you're not already seeing one, to work out why you SI and help you through those feelings.As ickydog said, ice can help alleviate some urges, snapping a rubber band on your arm might help too. What helps me is using red food die and poor it out in the sink or down my arm, just seeing something like blood calms my nerves.
I wish you the best