Will my scores be cancelled if I wrote on the SAT answer sheet?

If I have to skip a question on the SAT, I write the number of the question next to the section on the answer sheet to remind myself to come back to it. I forgot to erase a couple of these numbers before I turned in my test. I didn't know before the test that you aren't supposed to make stray marks on the answer sheet. :/ Will the computer be able to score my test or did I totally sabotage myself?

This doesn't sound like an issue that will result in cancellation of your scores. The computer is only checking the circles where you grid in your answers, so if you wrote on the edge of the sheet it shouldn't matter. The instructions on the test encourage students not to make stray marks because an accidental pencil mark in one of the circles could be read incorrectly by the computer as a student's real answer to that question. It doesn't sound like you're in danger of this happening. Still, I would encourage you to avoid writing on your answer sheet in the future!