I know, I have done this too. In fact on one particular quilt, I was such a ditz that I cut it too small not once, but twice! Can you believe that? There are some days that you need to just walk away from a project when you are in a hurry and making too many mistakes.
As frustrating and scary as it is, as everyone says just add an extra length of binding. If you are careful, you won't even be able to tell that it is there.