In my personal experience, you get motivation from successfully doing the work rather than waiting for some miraculous surge of energy. Continuing the analogy, burnout is similar to overtraining. Burnout can be a sign that we have pushed our brains too far, but we have to be careful. Just because your body is sore doesn’t mean you overtrained, just as if you are a bit tired you haven’t overworked. Of course, if these symptoms persist, we have to take note of it. But while we can overwork our brains, we have to use it to get stronger. I felt this during all of my training for the half-ironman. If I trained the day before, it became easier to train the next day. But I had to drag myself to the gym if I even missed one training session. I'd take the mentality of “I skipped the last one, I’ll do it tomorrow.” Lack of self-control is a direct result of inaction.
Instead of blaming genetics, limited willpower, or circumstances, sometimes you have to just act, without thinking too much. As David Goggins has said, “you have to train to overtrain.”