NextLevel has actually given the important info. David added to it.
If you are using your upper arm so much that your FH bothers your shoulder, you need to fix your technique.
If you are damaging your shoulder as a result of your FH technique, you need to fix your technique.
Using Euro rubbers with catapult will help you use the rubber more and the shoulder less. So that may be worth doing. But nothing can replace fixing the technique.
And if your FH technique is damaging your shoulder (which it is or you would not feel pain from playing) then no amount of stretches will help and no amount of rotator cuff muscle strengthening will help.
Those things will only help if you also fix the technique. I have played TT for 7 hours straight and it did not bother my shoulder. If 3 hours bothers your shoulder, you need to fix your FH technique.
As David said, in a FH, the upper arm should not move too much. It should be mostly weight transfer, core rotation, forearm snap and wrist from whip mechanics.
I can post a whole arsenal of photos for stretching your shoulder. I would also encourage you to do a search for exercises for strengthening your rotator cuff muscles. Because almost all adults would benefit from that and most shoulder conditions are helped by rotator cuff strengthening, even when that is not the underlying issue.
If you take ibuprofen because your head hurts, it might do something. But if you keep banging your head into a wall, your head will still hurt and over time it will hurt more and more.
The same is true for your shoulder. If you do stuff to make it feel better and not hurt as much, but continue to do the thing that is causing the trauma to your shoulder, over time it will become more serious in spite of efforts to alleviate the pain after you cause the trauma.
I hope you understand what I am saying and that this helps.
Without fixing the mechanics that are causing the problem, other things that are good for your shoulder will not be enough to help. You have to stop doing the motion that is causing the pain and the damage to your shoulder first. Pain is our body's warning sign. It is there because you are causing some damage to the shoulder. Not so much that it won't get better. But enough so it is worth changing things.
And I guarantee that the movement that is causing the pain in your shoulder is also causing your FH to be not as good as it could be.
When you fix your technique, the pain will go away and your FH will be a much better weapon for you.
But some stretches won't hurt as long as you also start trying to figure out what needs to change in your FH technique.
Good luck.
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