I know how you feel, I am 19 and I feel like crap, but I am chronically depressed, so I might be in a different position. Did you try seeing if you suffer from depression? I have/had self loathing issues and depression, which lead to procrastination (I wasn't lazy, I would be genuinely busy doing a billion things void of any productive outcome). The procrastination lead to obvious under performance, etc etc. It was a form of self targeted, passive, destruction (look at me being all sciency).

P.S: I was on pills until recently, when my psychiatrist told me I can stop taking them to see if I still need them or not (they also make me very sick for a short while, roughly 20 to 60 minutes on average), and I am starting to fall into my old and bad habits, again. Just thought I'd add that if anyone's wondering about how I feel.