First, have you tried to wet block it and stretch out the neck as much as possible? Soak it completely, squeeze out the excess water, then try to shape it the best you can.
You can undo a bind off edge, but not the cast on edge. If blocking doesn't work, the only way to correct this is to cut off the cast on edge, pick up the live stitches, re-knit a row or two, then bind off. If you're feeling adventurous and want to try it, this link will help you. Good luck!