I have had unproductive patches too as an independent. right now I am going through a reasonably productive time. There are times when the inspiration just kind of flows, and even hard problems end up being easy when confronted. There are other times when simple problems end up hard. There is some natural oscillation between the two. I have started to learn to take advantage of the pattern. Take down time when I need it (that's a hard thing to do as an independent) and work hard a lot of the rest of the time, even if it's only 6 hrs a day of hard programming (and probably 4 hours a day of other crap, and another 2 hours a day of still other stuff).
I hit burnout and was less productive for a long time.
It's taken about five years, but I'm almost back to full productivity. I'm basically back to normal, or better, in my day-to-day work, but I still have almost nothing in the way of progress on side projects compared to before.
Advice:
1. Pace yourself, get some work done, then go make coffee/food, or in my case, go for a motorcycle ride/work out at the gym.
2. Exercise. Hard. No walking on the treadmill shit. Start strength training and do sprints for conditioning/cardio. Our bodies weren't meant to languish in front of a computer and tend to throw a snit-fit if you don't satisfy them.
3. Have a hobby/hobbies that don't involve a computer. As stated before, for me these are strength training and the motorcycle.
4. Read a lot.
5. Focus on actionable, bite-sized amounts of work. Don't think about macro, long-term, or broad scope stuff. You'll just get overwhelmed. Let the dopamine hits come rapid-fire as you check things off in quick-succession throughout the day.
100% agreed with all of the above, except I would add one caveat to #5....
I think one does need to focus on macro, long-term, and broad scoped stuff a bit. This doesn't mean keeping it as a goal (which I think is your point) but it does mean setting aside time to plan, think about the long-term, re-evaluate where you are from time to time, etc. At the same time these plans should be shelved once complete and only reviewed periodically. The point of such planning is to think about the long-term not map out how you are going to get there. As Eisenhower said, "Plans are nothing. Planning is everything."
