Great post. I can relate; I've struggled with it for years. But last year I quit Twitter and Reddit for good. I've quit commenting on news websites. HN is the only place I time-waste anymore. And at least HN is obliquely related to what I do for a living and to my side projects, and it's frequently educational. I'd say I learn something new here that's actually useful to my work or gives me new ideas almost every time I come on.
I got to the point where most other things online - like the diet coke in your analogy - just became unpalatable. One day it just happened. The thought of going on Twitter sounded gross and contrary to how I view myself. I unfollowed everything. On the rare occasions I have the impulse to check it now, I only have to stop for an instant and remind myself: It's a cesspool. And I don't want to be part of it. This is not who I want to be.
In particular, I think, it was the anger and meaninglessness of random people's uninformed opinions, and what it did to me psychologically. Idiots yelling at idiots all day, all night. I was brought up to always argue for what I thought was right, but the rage machine finally just overloaded my circuits. The urge to jump into that and "fix the world" with my oh-so-special opinion just evaporated. The feeling I have toward Twitter now is finally a pleasant numbness, and a vague sadness that it exists.
I also remembered a compliment an ex-girlfriend paid me once, long before the days of time-wasting on the internet, when I was just a kid who never watched TV. She said I never wasted any motion in anything I did. I think about that, and it reminds me that efficiency is more central to my personality than any bad habits that have edged their way in since.
One final thing about side projects. It's hard to sit down at a blank page, and you'll look for almost any way to procrastinate. But once you view it as a problem or something that needs to be fixed, you'll spend all night on it without hesitation. I've found that the best way to motivate myself is to start showing it early to people, and have an audience in mind. Wanting to show friends your progress has a wonderful effect, and lets you know if you're on the right track.
tl;dr, I think you're onto something about associating the good habits with your identity, and the bad ones with something anathema to your being. Don't be too hard on yourself; the human brain was not made for this environment.