Reducing stiffness in strumming hand

Hi there, I’m playing for over a year now, been playing on both classic and electric guitars, and can consider myself an intermediate player.

While I don’t have any difficulties with soloing, I do have a problem with rhythm.

My right arm is really stiff, like REALLY stiff. I have difficulties playing fast and flowing strumming patterns because of that stiffness. No matter how relaxed I’m trying to be, I still end up messing up the rhythm or getting stiff again.

Some of the songs that I tried to play and faced difficulties with the rhythm are - another brick in the wall pt 2 (Pink Floyd), heaven knows I’m miserable now (Smiths) and even Smells like teen spirit which is pretty embarrassing considering I can play much more difficult pieces. It’s just that I truly have a hard time loosing up my right arm.

Any tips on how to improve that?