Start over. If you are being graded the teacher will know you hacked it.
If you shave, split apart a new Gillette razor (or whatever your brand of multibladed cartridge razor blade might be) to get some thin, super sharp blades.
Cut the miters short so the center doesn't fall out. Finish to the miter corner by hand with the Gillette blade. You will be amazed at how clean the corners come out. And it will impress the teacher.