I tried for a while to make those small changes, but I always found it too hard to do, until I finally just decided to cut out all animal products one night, and I never really went back.

I think the difference was how I framed it, mentally. I always saw it as an act of willpower to not eat animal products, like I have to overcome my cravings in the same way I would if I was cutting calories. But quitting animal products altogether allowed me to frame it differently for myself -- instead of telling myself "I shouldn't eat this", I can just say "I don't eat this." Like, it's not on the table as something I have to consider. I don't even have to recognize animal products as food.

Maybe if you cut things out one at a time you could do a similar thing. Though one problem is that it's a series of changes and commitments you have to make, instead of just one thing. I feel like that could be harder, depending on who you are.