You are not an idiot, don't be so hard on yourself. We have all done it and will probably do it again in the future. I just "did the math" on a quilt for my son and added a border of six inches and thought I was being so smart to add the six inches for my binding. Only I didn't remember that the six inches was going all the way around the quilt so had to piece to make it work. So piece together a square and as everyone has told you, when it is quilted you won't even see it.