On mitered corners it is important to stop sewing exactly at the 1/4 inch from the corner mark. This allow you to fold the main body of the block or quilt on the 45º and draw a true 45º line on the border to be mitered. When you sew the miter, you should stop at the same 1/4 inch mark on the inside edge. With all the pieces stopped at the 1/4 inch mark in the corner you can iron the corner nice and flat without any bulk. I'm glad you got it worked out and maybe the next one will be easier.