There is not a single person on this planet who stays motivated for years on end. Motivation is a brief moment, and you'll barely even be started by the time it's over. Fitness is a slow, tedious grind. If you rely on motivation for your personal growth, you will disappoint yourself. You'll think that it's because you just weren't motivated enough, that other people who succeed are just more motivated, they're always motivated, they're excited to get out there every single day and work at improving themselves.
I woke up this morning after four hours of sleep, sat around in bed trying to go back to sleep for an hour or two, and then reluctantly got up. How motivated do you think I was? I was pretty motivated to sit around and do nothing. I certainly didn't want to get dressed and drag my ass out to the gym so I could watch my body fail to do things that I easily accomplished two days ago. No points for guessing what I did today.
The workout predictably sucked and I cut it around 70% of the way through my normal routine, but I still got a full body workout done. Because it doesn't matter what I feel like today, it matters that today is a workout day and that means I go work out. No ambiguity about it.
Motivation sucks. Let it go.
As someone whose brain struggles with actually doing things, I never liked the "be disciplined" rhetoric.
Instead, I frame it as "you've got to practice going to the gym, and you will fail over and over, but as you get better at going to the gym you'll get a little more consistent."
That way, I could see my progress, and missed weeks or even months weren't a setback, just a sign I needed to practice again.
When I can't get myself to get dressed for the gym, I can often get myself to do some psuedo planche pushups or a few archer pullups, and that's practice too.
Discipline is hard, and you're going to have to practice it all the time.