[Lisa~]

OK, so you're doing the exercise correctly, you're just extremely stiff into internal shoulder rotation. You'll want to work on lengthening your pec/pec minor. Julie gave you some great advice. And dynamic work to lengthen the pec/pec minor would help too, like the PNF Diagonals from I/O. I'd guess there's some thoracic kyphosis happening as well. So foam roller type work to extend the thoracic spine would help too.

If you're that tight, then you probably need to work on lengthening this area multiple times a day for a while until you begin to see some improvement. I/O also has a section in the manual from Bill Hartman called Behavior Modification. There are a lot of movements to do during the day at your desk at work that will assist your efforts to improve your issue.