The easiest way to try and correct this is to think space between the ears and shoulders. The muscles marked in green picture 1.3 are pulling the shoulder blades down and the chest becomes open at the front. In turn, this will help with breathing, an essential part of cycling.
1.3 Green muscle are drawing the shoulders down and acting as a anchor for the shoulders, taking the pressure off the red muscles around the neck.
Please note the differences between picture 1.2 and 1.3, they are subtle but very important in the reduction of shoulder pain. Please remember that all of the changes you make should never be forced and done gently. You need to build these movements up. Do not go out and try to ride in this position for 1 or 2 hours, your muscles will fatigue. Drill while on bike: 10min normal shoulder posture, then check for steps 1,2,and 3 and make sure you ride correctly for 1-2 mins. Relax and repeat this throughout the ride. On your next rides increase the drill time by 30 sec until you can hold it for the entire ride. If you feel your upper back muscles need some help with strength, here is a exercise that will help strengthen the correct muscle. If pain persists, please seek the help of your therapist and have a professional check your position.