Reward could be anything. The feeling of a job well done (easy to miss if the project is a failure, or if management pivots), it could be the expectation of career advancement, it could be soft recognition by peers... And is dependent on the individual and project. You could even have an outward success and a pay raise but if you wanted your peers to love you and they didn't.... Burnout. You could even have an easy and unpressured job and burn out if it's not providing the rewards you expect.
In any case the disconnect between effort and reward teaches your brain to associate effort with failure and the fact that the common. "take a break" advice failed for you should not be surprising, because I think that doesn't work in general: it doesn't reassociate effort with expected reward.
I feel like a lot of people here are using the topic of burnout to hoist their opinions about American capitalism or whatever, but I honestly don't believe that this is the root cause. Plenty of people work their asses off and are happy to do so because it can be its own reward, or, they know what they want and know how to get it after each brutal push of effort. But not falling victim to burnout takes self-awareness, or good managers (capitalist systems or otherwise - e.g. academia or military) and both of those are in short supply, blaming capitalism is much easier.