How do I handle things falling apart at the last minute?

You can carry most things to the finish. It is the ones that go sideways right at the end that throw you, because you can see the finish line and then the ground shifts under it. That late collapse stings more than an early one, precisely because you had let yourself believe it was done. The closer to done, the harder the fall feels. But notice what the late unraveling actually is: not a sign you failed, but a sign that the end of anything is where the pressure is highest and the surprises come out. Everyone involved is tired, the stakes are clearest, and the hidden problems finally surface. Things do not go wrong at the finish because you are careless. They go wrong there because that is where the hard part was hiding all along. So change what you expect of yourself in that final stretch. Do not treat almost done as done; treat it as the most dangerous part, and keep something in reserve for it. Before the finish, ask one question: what is the one thing most likely to blow up here, and what will I do if it does. You cannot stop the surprises. You can stop being surprised by them.