I have a 23 year old retired polo horse. She has arthritis too. Like your horse, she does well during the warm months but the winter is really hard on her.
What helps her is getting her moving. I have noticed that even though she is turned out 24/7, when it is really cold she doesn't move much so I make sure I pull,her out of her field at least 3 days a week amd slowly hand walk her all over the farm to get her joints moving. It helps a lot.
I also have her on joint supplements. I started with Smartpak Smartflex II but have recently switched to the Dumor brand at tractor supply.
Just counting the days until warmer weather gets here.