How do I handle the pieces that push past my limits?

You have the technique now, and the demanding pieces still find the edge of what your body can do. That is not a sign you fell short. It is a sign you are working at the height of the form, where everyone, even the greatest, meets a limit. The pieces are supposed to expose it. That is what they are for. A limit is not a wall; it is a current edge. Some of it will move with patient, intelligent work, and some of it is simply the shape of your particular body, which is not a defect but a fact. The art is learning which is which, so you push where pushing helps and stop fighting what will not change. One step: take the piece that exposes you most and break it into the single hardest moment. Work that one moment with full attention, slowly, apart from the rest. Limits usually give way not to force but to precise, repeated, honest attention on the exact place they show up.