I had exactly the same accident. I was stressed because a beloved grandson was in the ICU and was cutting pieces to take to the hospital with me. Ruler slipped and cut right along side my nail like yours did. Another grandson took me to the drugstore and bought some kind of bandages that you leave on until they fall off on their own. I did that and when it finally came off my finger was well! Tender but all healed. Am extremely careful now!