I mostly don't go down the "motivational quotes" path, but there's one that has had a huge impact on my ability to Get Things Started.
It's simple, and it's not 100% accurate, but using it as a mantra is responsible for more of my habit-formation successes than anything else.
I forget the exact phrasing of how I first saw it, but the phrase "don't make the mistake of thinking that tomorrow you will be a different person than you are today" is the one thing that cuts through my natural procrastination urges and get me moving on self-improvement goals.
I say it's not 100% accurate because there's evidence (ironically I can't find where I read it, take that collector's fallacy, I should have saved it) that starting a new habit is easier if you do it at a moment of major life change, like starting a new job or having a kid.
In my case, my kid starting half-day daycare was a disruptive change (the "good" kind of "disrupt") that made for a good time to try to build new habits.
But those kinds of opportunities to trick your brain don't come along often, so in the meantime, I rely on my mantra:
"Don't make the mistake of thinking that tomorrow you will be a different person than you are today" means that when I think to myself "ugh I should start stretching more often" I have created a little trigger in my brain that means my NEXT IMMEDIATE THOUGHT is always...
...if I think I should start doing something impt, that I want to become a new habit, tomorrow — why not start now?
Why not stretch immediately? Why not fit in some gym time tonight?
I don't mean one-off tasks; I'd never finish anything if I tried to do everything. But habits?
I went to the gym last night and did half an hour on the treadmill and some stretches. I got out of the habit of going to the gym because of the pandemic and some postpartum health problems, but there's no better time than now to start fitting that into my schedule.
Because if I don't think it's important enough to fit it in RIGHT NOW, when I actually have the idea, when it feels fresh and new and exciting, when will I ever?
Obviously sometimes immediacy isn't feasible (like literally right now I can't, because in 10min I need to feed my kid), but taking a concrete step towards bringing the idea into reality is always helpful for me. Once I start something, I usually don't have problems finishing.
Some people are really good at forming habits. I'm not. Even something as simple as brushing my teeth at night is something that I can very easily convince myself "isn't important" because my brain is good at optimizing ROI and weaseling out of things I don't NEED to do.
The tricks my brain plays are mostly beneficial to me; I am very efficient, I don't waste time on low-reward activities, and I constantly re-evaluate things which lets me avoid the 5-monkeys-experiment problem: intersol.ca/news/organizational-culture-and-the-5-monkeys-experiment/
But the downside of decisive critical thinking about everything is that it's very hard for me to turn my brain off and let good habits go on autopilot.
I know a lot of people who eat essentially the same meals every day, who do their chores like clockwork, etc.
That's not me.
I need mental tricks to be productive, and one of the tricks I learned during therapy in my 20s after a major depressive episode (I'm ok now!) was how to break dangerous thought-cycles and replace them with useful self-talk.
I will not be a different person tomorrow.