https://www.active.com/cycling/articles/exercises-to-treat-shoulder-and-neck-pain-from-cycling
These exercises did wonders for me, a few times a day for two days and cured. Also angled my bars up 10 degrees
I needed NHS physio to address this exact problem which was causing me a great deal of discomfort. Strangely the only time I felt comfortable was on my bike.
With the exception of No.10 these are exactly the exercises I was given by the physio. I was advised to do 6, 7, 8 and 9 first in that order as a warm up. I was also advised to warm the muscles first before the exercises, this was after a shower, or with a hot water bottle or wheat bag placed on the neck for ten minutes.
Ideally the exercises are done five times a day. I managed three and my physio was happy with this. For me there was a significant immediate improvement but it took several months to "cure" the problem.
I had a bike fit by a second physio who offered the service. I felt the combination of qualified physio and very experienced cyclist put him in a good place to understand my issue. The change in position was beneficial in several ways.
I was given one additional stretch which was the hardest to do.Look down to the left with your chin touching your chest, place right hand on back of head and push head forward and down to extend the stretch, hold for five seconds. Repeat five times and then on the right side.