Light on Yoga by Iyengar has some great tips for this posture in his book that may be helpful. Also you could explore drawing your legs in toward the mid line of your body thus balancing out your pelvis. Once your pelvis is balance there will be more space for your shoulder to move as it will not be pulled back as far. This should give you more space to level your clavicles and open your upper chest. Practice this pelvic opening and strengthening on the floor first where you are fully supported. The leg that is in front and bent, draw your toes back toward your shin and isometrically pull your leg back towards your body and in towards your mid line. Do a similar action with your back straight leg. Curl your toes under lift your upper inner thigh toward the ceiling and isometrically drag your leg towards your pelvis and in towards your mid line. Take a few deep breaths here then explore opening with your exhale and creating length in your legs and pulling in towards your mid line with your inhale. Breath into your lower back to create space there and from this new space open your upper chest woking towards balancing your shoulders.
