“safely” cutting

I’m kind of scared because whenever I cut there’s usually a string of something that I can’t cut through even if I multi swipe. I’m worried it’s like a vein or something. Idk how nerves and veins work though. it could just be skin but sometimes after I cut my whole arm feels numb(the longest this has lasted is a 2 weeks) so is there anyway to avoid cutting veins and stuff like that?