My girlfriend is depressed and self harms. Should I tell someone or could it make her feel worse. She doesn't care weather she lives or dies and I don't know how to help her. Any additional advice is welcome as I don't know how to help her. Thanks?

If you truly care for her you will tell someone. Yes, there's a big chance she will be mad at you (at least on the beginning) but that's her illness talking. Think about it, do you think anyone is living any kind if a quality of life if they don't care whether they live or die? Yes. Tell someone and if they don't act, tell someone else. 

Your relationship sounds depressing.  She's dragging you down as well as herself. Get her help or get out.  If you value yourself, you will save yourself.  There many happy people you can enjoy life with.