Every last one of us has made a mistake and been extremely upset by it. You did everything else by the book, you had a doctor look at the patient, you filed an incident report. The fact that you feel bad shows your true potential of being a good nurse, not that you made a mistake. I bet you won't forget the tourniquet again! We are human, and have stressful busy jobs. Try not to be so hard on yourself, and good luck.