In case you haven't been avidly following my Practice Tracks blog series, I've got news for you. I'm working on some pretty tough music. I'm even practicing one section in Art Tatum's 'Tiger Rag' that I first started working on over two years ago. It's a good thing I don't have much hair, because at this point I might be pulling it out.
The good news is that, especially over the past few months, I've made very slow but steady progress. Part of that has been my improved ability to break down difficult passages even further into small, incremental steps, but an equally important part has been knowing when to stop and rest.
Really complex tasks don't just demand a lot of work, they also require time to process the new skills you're developing. It's imperative to know when to quit working for the day, rest, and come back tomorrow. You can't accomplish everything at once. Make measurable progress, then give yourself a break.