sustraley, your borders are off beginning with the white strip, & yes you'll need to be doing some unsewing, measuring. Wish I was closer to help you fix it, so hard to explain here. But remove borders along with the white strip, remeasure all of them, sew all of them back together, measure quilt and do same with border strips, placing pins at center mark & at each end corresponding with width of quilt. Attach to quilt leaving a 1/4" at either end. I then fold quilt aligning top border with the side border making sure everything is even, making sure strips match, using ruler with 45 degree angle and draw my miter, pin & sew beginning from the quilt out to the corner. It all comes with error & practice, practice. I almost always miter my corners because I think it makes the quilt look more professional, more finished, if that is possible. I hope this helps along with the tips from others and isn't too overwhelming. Once you learn how to do miter corners & find out how easy they really are, that is all you'll want to use in the future. Good luck, it will turn out beautiful.