Just because you haven't gotten an infection in all this time doesn't mean that you wont. You've been playing russian roulette here. The mouth is a disgusting place with loads of bacteria and acts as a breeding ground for all sorts of nasty things. So having open sores and such there can lead to infections just due to proximity of your mouth, and not the fact that the injury itself is serious.

I'm 20 as well so if you ever want to talk about anything feel free. I've been self-injuring for ten nearly 11 years now and although what you do isn't a common or stereotypical form it is still SI. Find a counsellor or someone and talk this over with.