Was it a one-time sudden realization, a nagging feeling you couldn't get rid of, or a back-and-forth process?
For me, it was 5 or so years of back-and-forth waffling between vegan and "reducitarian/VAH" before a very strong dose of magic mushrooms a few years ago gave me a stern kick in the ass about my own ethics and hypocrisy, which is how I finally stayed vegan. The same trip was the beginning of my political radicalization left as well.
I got an institutional start. When I was a backpacking guide they asked me for my meal prefs, so I wrote down vegan because I knew it was the right thing to do, even though I was eating meat prior. had no more choice in the matter for 6 months, and then it was easy to keep going.