Why is pain addictive? i mean i hurt or i dont know cut myself for it especially my nails to the extent it becomes paronychia and swollen

Get help: Repeated cutting is self-harm which you say you can't stop because "it's addicting" - as I understand your comment. Please talk to your parents, tell them what's going on and that you need the help of a therapist or psychiatrist. Have them call your pediatrician or pcp to meet and discuss for guidance and a referral. I hope this helps you. Be safe.