Been there as well
I've completed a few big projects in my time, with thankfully only minor niggles to correct in the new version.
What gets me the most is that I know that, however much preparation and planning and caution I employ, in the end, on launch day, there will always be one user who will have an incredible problem nobody else does. And when I'm confronted with it, I then have to tie my brain into a knot in order to find a solution that doesn't break it for everyone else.
And, often, that one user will be a VIP or otherwise important person in the company, so the solution just has to be found ASAP.
It's almost as exhausting as the entire rest of the project.