Hi ginger1776,
If you are snapping the band hard enough to create welts, I would have to assume that you're snapping the band with quite a bit of force, right? I think this method is supposed to be for a little sting at most.
Personally, it never worked for me. Something that DID work better (temporarily) was the ice cube method: pressing an ice cube against my wrist hard until it melted. You'd be surprised how much that could hurt, and it doesn't leave a mark.
That being said, I have found that the only way I was able to achieve a long period from SI (over 6 months now) was to focus on WHY I was self-injuring, as opposed to distracting myself with less severe pain. Because for me, it left me wanting more.
You have my deepest condolences.

Do you have someone close to you, a family member, friend, or therapist, that you can talk to about your struggles with wanting to self-harm?