Why does work always fill up my free time?

You finish early, a gap opens, and work quietly floods in to fill it. You have never once managed to use that extra time for something else. That is not a personal failing. It is what happens when work is the only thing with a clear claim on your hours and nothing else is waiting to be chosen. Empty time does not stay empty. It goes to whatever is loudest and most ready to hand, and work is always both. If you want a spare hour to become something other than more work, that something has to already exist before the gap opens. A vague intention to relax will lose every time to a concrete task sitting in your inbox. So give the free time a shape in advance. Decide now what the next open hour is for, and make it specific: a walk at the river, ten pages of a book, coffee with a friend already on the calendar. Protect it like it is an appointment, because to work it looks like empty ground to take. You are simply planting something there first.