Hey, I certainly don't mean it to sound like I'm giving you shit. (I agree - ALWAYS play through a song.) My right thumb bends back past a right angle because of whacking it on the pickup of my bass for a bunch of years. It hurts when it rains and everything.
It's just more like, take better care of yourself and learn to not hurt yourself on the instrument. Change string gauge, get the neck adjusted, hold the guitar differently, whatever. If you're really doing the Hetfield super-downpick thing, I'd say switch to bass picks and just plain old get your fingers further awau from the strings. It's not about holding back, maybe more about focusing.
I enjoy sloppy playing as much as anybody. I guess I just don't think extreme bodily harm is a part of it, is all. I reckon we're not playing the same type of music, though, so that's fine. I play light "easy" jazz.