Also, like me, you may have to accept that you are getting older and can't rely quite so much on having that super-sharp working memory to hold an entire codebase in your head and stay focused for hours on end. A lot of my burnout came from putting unreasonable expectations on myself. Once I started taking a lot more notes, drawing a lot more diagrams, and asking a lot more questions I actually found I became more efficient than when I was relying on mental capacity alone. And it became much easier to take breaks and tolerate interruptions.