What dangers, exactly, exist in postponing treatment for self-injury?

Well, it depends on what kind of self injury you do. If its superficial wounding, like cutting or burning, then there wont really be any physical long term harm. But if you're shoving things into your skin, ingesting things, or forcing yourself to vomit, then there could be muscle damage, poisoning, or damage to the esophagus. But there are alternatives to self injury. Like I used to cut, and, while I wont explain my reasons here, I did it more to see the blood than to inflict pain, and now I donate blood. I'd try quiting if I were you.