I just went through this last week, and I gave in and cut after going about 6 weeks w/no SI prior to that. I know how hard it is to keep from doing it, especially once those thoughts set in & just won't go away. And then, once you do it and the initial 'high' has worn off, the guilt can set it.
When I talked to my T about this last Mon, after my SI incidents on Sun, she said not to be so hard on myself, it was just an thing, I did it, and it doesn't have to be more than that. Somebody here on PC said something that has really stuck with me, too, and that is, it doesn't *have* to be a crisis, it's only a crisis if I MAKE it a crisis, if I want to I can make it just an "oops", just a moment, you know?
So, try not to spend a lot of time with those guilt feelings. You fell down, everybody does sometimes. You can start again from here. First with one hour of not cutting, then 2 then, and on like that.
Hope that helps a little bit.