Forhand topspin

Hi i have a big Problem in my forhand topspin that i somehow can Not keep relaxed and only tense up in the contact Point. The think is even at warm Up If i try to Play topspins with max Speed and my opponent Blocks fast Back many Balls there i Mostly shorten the stroke because the ball is coming fast Back but also If the ball comes fast Back i can Not somehow relax the right way i feel like going immediately into the shot i dont have this phase of Reaktion and this IS the Problem why my arm fatique fast. Do you know how i can fix this maybe also some Shadow Drills to make IT Automatic especially If my opponent Blocks the Ball fast Back to Back ?