How do I cope when my body is wearing out from the trade?

Your walls will stand for a hundred years, and your hands and back are paying for every one of them. That is the hard bargain of skilled physical work. You made something lasting, and it is costing you your body to do it. Being proud of the work and hurting from it are both true at once. You do not have to choose between them. But your body is not an endless supply, and honoring the craft means protecting the man who does it, not just the wall he leaves behind. Start taking one thing seriously that you have been ignoring, the lifting, the stretching, the rest between jobs. Small care now buys you years of working later. The walls will outlast you either way. The point is to not let them outlast you too soon.