Along with all the other advice offered here, Which piece did you have on top when you sewed the outer border on to the narrow second border? I have learned that I have much better luck if I keep the border piece on the top when sewing--in this case, having the wide outer border on the top when attaching to the second border.
I also measure both edges and thru the center and use an average of those 3 measurements for my border.
And finally, I know it is more cumbersome, but I would have added each separate border to the top, not sewn them together first.