why is it so difficult to cry? I just can't do it. I want to, but I can't.

I'm a 20-year-old male who's trying to get through engineering school (which I'm not the least bit passionate about). My grades are slipping, my anxiety and depression are skyrocketing, I can't stop telling myself that one day my girlfriend (the love of my life) will realize how much of a piece of shit I am and leave me. 

I'm so negative all the time. I just want to sleep my life away.......... /die