How do I keep my writing strong across a whole long piece?

You can write well. That is real, and it is the hard part done. What is left is a different skill from writing a good sentence: it is stamina and shape, holding a whole thing together. Almost no one has both at once. The fact that the long work defeats you does not undo the good sentences. A whole piece falls apart mostly when it is carried all in the head. Structure is what carries it instead. Before you polish, get the bones down, even roughly: what each part is doing and why one leads to the next. When the shape is on the page, quality stops leaking out in the middle, because you always know where you are. Try drafting your next long piece ugly and complete before you make any part beautiful. Finish the whole rough shape first, then raise the level pass by pass. You already know how to make writing good. You just need to make it good after the frame is standing.