How can I grow my nails long without having the temptation to scratch myself?

I used to cut myself with a blade but I was forced to stop because of summer and my mom found out when she saw them on my legs. I've always had the urge since so when I'm mad I use my nails to pinch my other fingers. I always cut my nails because if they were long I would have marks all over me. I had to grow my nails out for a dance at my school and the dance was horrible because I'm self conscious and that's why I don't dance. The whole time I just watched everyone dance and I just wanted to cry but it didn't want to do it in front of everyone do I pinched myself harder than ever. I have red marks on my palms and blisters on my fingers from using my other fingers to hurt myself. When I did it I was semi concious of it but it stopped my from crying. I decided I liked my nails long but I don't know how I would have them long without building the habit of pinching myself. I do it at other times but not as bad. How can I grow my nails long without having the temptation to scratch myself?