You said it, my exact sentiments. I think when you are playing something where there is a rating/score and you can win or lose, it's very difficult to not focus on (or sometimes be obsessed with) results. When it's going up, there is all the motivation in the world, but when it isn't...deflation.
I love seeing my puzzle rating go up, but I certainly do not have a problem with tilt doing puzzles on a piece of paper. That said, if I couldn't see tangible improvement by higher ratings/winning, I doubt I would play. I enjoy the game but I like to compete and win.
I do not do endurance sports, but I do weight train regularly. Particularly as I've crossed the 40 threshold, I've learned two things about my training: (1) I have to back off and fully recover regularly, and (2) changing up not just exercises by rep ranges can fuel progress.
To handle (1), for the past year, I've done deloads every ~7 weeks: Train with increasing intensity week-over-week for weeks 1-6 and then during the 7th week, dial back to lower than where I started. During that week I enjoy the easy training and plan how I want the next six weeks to look. Sometimes minimal changes, but the break takes me from near exhaustion to dying for the next Monday to get back into the gym and work hard.
To handle (2), even though my main focus is 10-20 reps, taking 6 weeks and doing super heavy is sometimes exactly what the body and mind need to 'desensitize' to the normal training. You may lose a little progress on your preferred rep range, but the motivation returns and also allows overuse injuries to heal.
I haven't figured that out for chess yet, but the above things have been on my mind for the past 24-hours. As I think about what to do next, I may build in 'easy' weeks (whatever that is) and also play time controls completely different than the last 6 months. All blitz and classical, forgo anything Rapid? Maybe. 
All my experience across life tells me if my motivation is playing hide-and-seek, I cannot find it by standing in the same spot, doing the same things.
I feel your pain; I am experiencing a similar issue with motivation. I think its partly a result of focusing too much on results and not enough on the process.
We know that ratings come and go and there are natural tides of ratings going up and down, but man it's sure hard to surf when the tide is on its way out.
I do a lot of ultra endurance sports, running, biking, paddling and I face a lot of burnout there from overworking. To combat that, I have started adopting a form of low heart training where nearly all my sessions are fairly easy (albeit long) and that has gone a long ways to curbing burnout.
I want to figure out some way to apply the same strategy to Chess. With running, biking, or swimming is easy. Just wear a heart rate monitor and stay below X heart rate.... How do I do that with chess?
If you have ideas, please hit me up!