When's a Decision Decided?

Why is it that just when you think you've decided something, you realize that you actually haven't? Somebody brings up a point you hadn't considered or money you saved for one thing suddenly needs to be used for something else entirely or you wake up one morning wondering why you're not fully satisfied with your decision. This has been happening to me a lot in the last several months and it's starting to make me wonder if I even know how to make decisions anymore.

One example of this is my potential move to Paris. I knew I wanted to go, I'd come up with a plan to make it happen and had more or less decided that grad school in Paris was my plan for 2013. Except that all the money I'd started to save for it had to go to sudden emergency expenses, like my taxes. And then the chances of getting a long stay visa seemed much less impossible than they had been before. And then I started to meet people with connections in Paris so that being a student was no longer the only way to make friends.

So now what? Now I'm back to square two: I have no idea what I'm doing when, but at least I have more options than I did back at square one a year ago. Still, my decision is now undecided, making me feel a bit lost.

When is a decision final for you? How do you know when to stop wondering about other options and sticking with what you decided?