Hi there, sejkhn. Sadly a fractured humerus is of the more difficult injuries to recover from, as you are finding. However, 12 weeks is, as yet, early days. Often there is residual stiffness and lack of range of movement no matter how hard you work at it.
However, you said you still have pain - are you taking medications for it? You could find that pain will hamper your movement even though you feel you are strong enough to push through it.
You have some weeks more of PT before you make a breakthrough. I hope you make it, my friend.