Here's a picture of the book with the sweater, so cute!
It looks to me that you did the armhole shaping at the middle. In directions, when it says to do the "left" armhole shaping, that means as if you were wearing it, not as the sweater faces you. So actually the left side is where your needle is now, and you would be getting ready to do the right side, where actually you did the left shaping on the right. Does that make sense? So in following the pattern, you would be getting ready to do the right armhole shaping (and binding off those four stitches in the middle for the collar, in which case it wouild make sense to join, bind off, and continue to knit to the right side of the sweater itself, not your right hand.
Now I'm wondering if perhaps you can still salvage the left armhole shaping you've done by taking out the row (looks like navy, or maybe the white row) just before you started the shaping since it's just one strand. Save your shaping, and put the bottom stitches on a needle, redo the row, and graft (Kitchener stitch) the left shaping in place. Someone else might be able to explain this better, like Ingrid. Anybody have any other ideas?