Every single time I'm about 99% of the way "done" with a talk, about 2-3 days before the scheduled run of said talk I second guess myself and almost completely rework/rewrite the talk from the ground up.
Almost every single time I do this I end up going right back to the talk I had ready to go. Instead of using that valuable time to get more comfortable with material I worked hard to put together, I freak out, question everything, try hard to make it better, then realize I'm spinning wheels.
I'm not sure why I do this. Insecurity? Maybe. Desire to do and be better? Probably.
Accept it for what it is. If you put in the time and the thought before hand, if you've practiced and you have material that comes from who you are and what you know AND it's honest, it's going to be OK.
Don't start over the day before your presentation. Stick to what you've got, feel it, trust it, sell it!