Oh my gosh, you are so NOT stupid ! I feel that way all the time but it's not about being smart or not smart or self control or not. once i almost cut my face, and i think sometimes about how freaking glad i am that i did not do that. I hope you didn't cut too deep and it will heal before anyone notices.
If you aren't in therapy, it is what you need. For some reason SI is one of the hardest things to beat. But i feel the way you do all the time;...but feeling about about ourselves only makes us feel worse and leads to more cutting. Hope you are okay...