Andy, I did make a conscious effort – but I feel like I had been making that effort for decades. I'm honestly not sure what created the shift.
The least helpful option for you is that, since childhood, I've always looked forward to getting old. (Maybe because my grandparents were full of interesting facts and fun things to do, maybe from exposure to the "wise woman" idea when I was younger, maybe from all that plus other input I don't even remember.) So maybe, now that I've turned 50, some kind of switch has flipped in my head and I'm fulfilling my own prophecy, as it were? 
Also not useful for you (I hope!): my almost-burnout may have helped, because it got me to take a long, hard look at why I always put other people's desires first. It started with client demands (I was always available, always accommodating, etc.) and expanded from there. When I got to my food choices, the insanity of "don't be any trouble to other people" hit me. I mean, talk about an orders-of-magnitude imbalance in importance. As I wrote in one of my stories, "Like I’m worrying about a stubbed toe when a bear’s just clawed off my right arm."
But I do have one useful, concrete datum. I stopped avoiding painful information. I was first exposed to the moral motivation for vegetarianism and veganism (and the accompanying graphic descriptions) over three decades ago. It made instant sense to me. But after that I went out of my way to avoid graphic descriptions, because they're so hard to bear. I figured I already knew what I needed to know, and I didn't need to keep exposing myself to that.
What that meant in practice, though, is that my conviction wavered in the face of what was present: other people's feelings and desires. So a couple of years ago, I decided to start seeking out information, hoping that would help me create an internal commitment so constantly present that it trumped my desire to be "no trouble" to others. And, indeed, that's what the shift feels like: being vegan has become easy because the reality behind the choice I'm making is now front and center in my mind all the time. So I think this tactic must have played a part (combined with the orders-of-magnitude realization I mentioned above). 
 Though, now that I'm here, I gotta tell ya, 50 doesn't feel old. I don't think I've hit "old" yet, actually.  But 50 still sounds old, so if there was a switch to be flipped, this last birthday sure could have triggered it.
 Obviously bodies do get old, but I'm pretty sure that mentally, I'll never get old. Which has opened my eyes (gradually, over the years) to the fact that all those white-haired wrinkly people out there are still the same people they always were: free-thinking or narrow-minded, experimental or straightlaced, fun to be with or boring as mud, ... Hitting 50 (or 60, or 70, or 80, or...) doesn't magically turn you into an old-fashioned granny. (This may have been obvious to everyone but me.)
 Some resources, if you don't already know them:
Michael Pollan, The Omnivore's Dilemma
Peter Singer, The Ethics of What We Eat
The documentary Cowspiracy
The documentary Live and Let Live