How can we ensure that we will finish everything on our to do list if something comes up?

This is a loaded question. The truth is that you can't always finish everything you planned, and assuming that you can will cause anxiety. Mentally reset, get some rest, and get ready for the next day.

Either note it down and keep working on your current task and come back to it later, or if it's more urgent then note what you're doing and do it first.
But this sounds nothing new or perfect just my little answer.