thats really bad. first of all you really should get that checked out. alot of things can go bad with an infection aside from the fact that its infected. cutting is easy to get addicted to and hard to stop. even when you do stop you always want it. honestly i am struggling with myself at the moment so i dont feel i can be much help but talk to somebody. i know it seems like no one will listen or no one will care but someone will. i think sometimes with cutting or harming yourself in any way, its hard to think about who you might be hurting especially in the moment. all you want to care about is you your pain and the satisfaction it seems to bring. i know its hard and i cant promise things will get easy. i thing you should start by taking baby steps and the first step would be getting your arm check out.