I'll be honest and say that there will be some times you can't stay motivated. And that's okay too. If you feel like you're gonna burn out, it doesn't hurt to take a short break every now and again. Been technically working on my story for four years before I started even publishing, had my first burnout after about six months, didn't draw for a few months. When I started again, I was able to look at my style/story with a fresher perspective and introduce some gradual changes to make it better (this was way before I published/found the forum though, so I was on my own for a bit without a real clue of what I was doing until July of this year)
But what's keeping me motivated now, despite having deadlines, is interacting with readers/creators. and occasionally doing something a little out of my comfort zone to challenge and/or spice up the routine. I've been drawing others' ocs as chickens for fun, which was difficult at first, but allowing myself to push a little bit has helped me learn and draw more efficiently, not to mention discovering a newfound enjoyment of the creation process, and I'm looking forward to incorporating these newfound skills into my upcoming works.
So, find something in your story/style/interactions that can be looked forward to, outside of milestones, comments, feedback that'll come along the way. I have a few parts of my story that, while are far off, I am excited to draw. Helps me get through those inevitable "boring" parts.