I always walk my binding around my quilts before starting to sew it on permanently. Ask me why I know to do that? Same problem you are having, but it was only once to realize look ahead so it doesn't happen again. Make enough binding so you can move it as needed when walking it around the quilt and also allow for you extra at each corner. Then I glue baste and heat set before sewing the one side down it is so much easier and looks so much nicer this way.
Good Luck
Suzy