How do I tackle big cleaning jobs without feeling overwhelmed?

You clean well day to day, so this is not about skill. It is about size. A huge job or a really tough mess hits you all at once, and your mind takes in the whole thing before your hands have moved. That flood of everything-at-once is the overwhelm, not the work itself. The fix is to stop looking at the whole and look only at the next square foot. Big jobs are not done by big effort. They are done by small parts, one after another. Pick a corner, a single zone, and let the rest of the room disappear until that part is finished. Before your next big job, break it into three chunks and start with the smallest one. Finishing something, anything, shifts the feeling from drowning to progress. You already know how to clean. You just need to meet the job one piece at a time.