Not sure if this tidbit of advice will help, but here's how I finished American Pixels. That was a 5+ year project, tons of excitement at the front-end, and then it turned into... work. And here's the problem, I have a family and a full-time job, so I kept hitting the snag where I'd work all day, spend time with the family, have a pocket of free time at night, and then... not have any energy left. That, or there'd be this sense of "yay, I have time to work on music... and oh no, I feel obligated to work on that project and I'm really not in the mood..."
So, that happened over and over, and occasionally I'd have a magic day off (holiday, whatever) where I'd get a wonderful 4 hours in a row to myself... and during those moments, I could tap into the creative juices and get it going again... but man, those little magical windows were rare. Like, once every few months rare. It got really frustrating seeing an album inch to completion in those sporadic bursts.
The answer for me: wake up earlier. I'd set my alarm for 4:30am, be in my studio by 5am, and get in a solid hour, hour and a half every morning. My brain wasn't yet tired from the day job, I was rested and fresh, and the stuff that seemed like work before now seemed fun again. And, it actually helped my happiness level, so even though I was sleepy, I was giddy at work considering the progress I made in the early morning hours. And then I just had to ride and chase that feeling until things were done. It was tough to get started, but then it became strangely addicting.