Hi Megan, don't despair. This is fixable.
I thought I'd jump in here whilst we're waiting for better advice than I can give. But I've looked at the pattern, and yes, it does seem that you should have kept knitting for about 5 more inches till you got to the same height as the shoulder.
This isn't too hard to fix. You've bound off the back that's too short, right? What you need to do is unravel the bind off and get those stitches onto a needle. You can gently tease out where the last bound off stitch is fastened, then the bind-off stitches will be easy to undo. If it was me, I'd just undo them one at a time, very slowly, and slip each onto my needle as I went.
But I don't know how experienced you are, and it might be a good idea to put in a lifeline just below that row in case you have problems catching the stitches. If I had time now - which I don't - I'd google a few links for you or try to find a video on this site about lifelines, but hopefully someone else will come up with those.
Once your bound off stitches are back on your needle its just a matter of knitting up a few more inches to match the height of the shoulder, bind off again, and all will be well.
Just stop stressing, this is not actually a phenomenal mistake, it would have been more of a pain if you'd gone too far and had to unravel your work. We all make errors like this at some time, but it CAN be fixed!