I understand exactly what you mean. I always took care of myself until around 7th or 8th grade when my severe depression began to manifest itself. In 9th and 10th grade I developed a pretty severe eating disorder and dropped over 40-50 pounds in a matter of two months...I still have that "anorexic" thinking constantly.
It wasn't until college when I really quit even trying to take care of myself. I indulged in a lot of alcohol and drugs, skipped classes, and ended up dropping out before my first semester was over. I probably just use not caring about myself as an excuse to keep destroying myself with alcohol and drugs. If I don't care about myself why bother ever being awake or sober?
If I'm awake, I don't want to be sober. If I'm sleeping, I don't want to wake up. I keep trying to work on it but ultimately, I don't care enough.