I'll be honest, if SI is kicking in right now all you can do is wait another day. The only people who could advise on how to overcome it overcame it themselves and aren't here right now. It sucks, a lot, I know, I tried last month and used THC to try to dull my senses but even that wasn't enough (mind you, I chose hypothermia which wasn't smart) and am still extremely angry at myself right now for it. Many people do say using alcohol or drugs can help beforehand - though I highly recommend against using alcohol or drugs (street drugs I mean) as a method as they almost always seem to not work and cause way more pain. I think in my case my method wasn't for me.
The only other thing I can offer is to rethink your method. The reason I had chosen hypothermia was because in the past I never could go through with the quick and violent ways like yours, so I thought something slower might do the trick. In my case, it didn't, but maybe you could find something that'd work out.