Your back has to last a lot longer than this job. Work like you know that.

A body is not a tool you get to replace. Lifting badly because it is faster, hauling more than you should to look capable, skipping the help you could have asked for, all of it borrows against decades you will still need. The job is temporary and your spine is not. Nobody else will carry the cost of that trade.

Learn the right way to lift the heaviest thing you handle, and use it even when the shift is behind. Ask for the second person, and use the cart. None of that is weakness, and all of it is cheaper than finding out later.