This phrase has operationalized so much in my life. It lowers the activation energy required to do hard things.
Sometimes you're tired.
Sometimes you're unmotivated.
Tell yourself: "Just get warmed up. If you're not feeling it after that you can stop."
Each time you'll go into it thinking that you'll quit after the warmup and call it a day. But it's a trick, a purposeful trick you play on yourself. You'll get warmed up and you'll be ready. Your fire will reignite and you'll be off to the races.
I first started practicing this technique implicitly as a missionary for the Church of Jesus Christ of Latter-Day Saints. I was given an hour every day to study scripture on my own. I had a companion who was, according to the missionary rules, within sight and sound of me almost 24/7. I was accountable to him.
If I was unmotivated to study it didn't matter, we were gonna sit down at our desks for one hour.
Now, I won't lie. There are days I utterly wasted. I'd sit there scrolling on Facebook or staring out the window. I eventually got tired of wasting time and figured out how to get myself out of neutral.
If I put my head down and just started reading, after 1 or 2 or sometimes 3 chapters I'd have found a topic that interested me and I'd be flying. All I had to do was grit my teeth for a few minutes and the rest was easy.
Later on I found that this technique applied wonderfully to the gym. If I got under the bar and did a few light sets, I'd activate my sympathetic nervous system enough to get me excited (or at least begrudgingly ready) for the working sets.
In college I've found that this even applies to class work. If you don't want to do it, start with something light. Review your notes, watch a YouTube video about it, glance over the lecture on it, and start moving.
Once you're wrung out, once your brain feels fried, get up and take a walk without any distractions. You can let your brain keep working in the background, but that's a topic for another day.
You have more energy, focus, and drive than you think you do.
Just get warmed up.