I cut out pop over two and a half years ago. I used to drink root beer like it was my oxygen, but I was surprised how easy it was to cut it out. I can have very good self control when I really want to. Diet pop is pretty bad for you actually, because if you drink too much then you can get aspartame poisoning, which is no fun at ALL. But as long as you're not drinking it excessively you shouldn't have much to worry about.
As for feeling like you're the fattest person in the world... I do that quite a lot. I hate mirrors and windows, and pretty much anything else that I can see my reflection in. I hate the way that my body is so unproportioned. And I hate the number that comes up when I step on the scales. I have a love-hate relationship with food that goes all the way back to when I was 6 years old. And I have never in my life maintained a proper balanced diet for more than two weeks. Food is a huge part of my life because weather I am eating, or I'm refusing to eat, I'm always obsessing over it.