Arthritis mostly depends on the weather. Your horse will have bad flare ups mostly in the winter and even some in the summer. She will be very achy. It affects their joints, makes them very sore, and sometimes sour to ride. You should really talk to your vet about giving her some joint supplements that she can have everyday that will help her.
Also, have you ever tried having her get a massage. My horse has arthritis and when I can afford it I try to get her a massage done to loosen up her tight muscles. Also, you should stretch her everyday.
Good Luck. Sorry about your horse.