The pull of some internal forces is too great to resist. When we are hungry, we eat. If there's no food then we make some. We find some. We don't think about motivation, or desire, or anything. It almost happens by itself. Because "I am hungry (for food). I will (eat food). I am (eating food)."
The same thing happens for anything else in life. Don't have motivation to find your dream job? Wait until you have zero money and zero options for your next meal. You'll make something happen soon enough. You'll find a way to survive.
Want to get in shape? Wait until your doctor says "You need to get exercise or else you're going to have a stroke and die." The sad thing is, even then some people continue their unhealthy ways right to the grave.
A great way to stay hungry is to see the benefit of the thing you're chasing. How good would it feel to be 10-15 percent bodyfat? What would shopping for clothes be like? What about looking in the mirror? What about sex?
Find your motivator. Stay hungry. But most importantly, hack your habits to support your hunger. Carve out recurring daily times, just as you would for lunch.
Me? I train M-F from 7-8am. No clients. No projects. Someone asks to train at 7:30 once a week? Sorry. That's me time.
Because I'm hungry.