Starch can be your friend...
If you stiffen the fabric it will help control the stretch; starch it, or apply school glue (washable), press when dry. You could then staystitch it if more control is needed--just sew a line of stitching about 1/8" from the edge (so it's invisible after seaming).
To get the stretched fabric back to shape wet it, let dry, then press, making sure not to stretch those bias areas. Be sure to take a before and after picture for later evidence of your efforts!