How do I handle large-scale cleanup projects that feel beyond me?

You can design the fixes you have faced, and now the very largest ones loom as a different kind of problem, one that seems to outscale everything you have built before. That defeat you feel is not incompetence. It is the honest gap between the work you have mastered and the next size up, which every expert meets eventually. Large does not mean fundamentally different; it means the same principles under more constraint, with less room for error and more moving parts. The way through is not a bigger burst of genius but breaking the giant problem into phases small enough to reason about one at a time. Take the large project that daunts you most and carve off just the first phase, the piece you do know how to design. Solve that, learn from it, and let it show you the next. And bring in someone who has worked at this scale before, because no one designs the biggest fixes alone. Start small inside the big, and the giant becomes a sequence.