If you are able I would immediately make plans to leave. There is no reason to have to put up with that especially if your horses aren't getting good feed.
I sympathize with you, I have boarded my horse as long as I have had her, have experianced some pretty bad things and have moved my horse alot. I've dealt with dirty water, no water, not enough hay, poor quality hay and moldy hay. My tack being used and not put back, my halters on other horses and I have even had tack stolen. I had trouble with grain not being fed when I provided it. Like you, there were horses being pastured in the outdoor arena that I have the right to use so I couldn't use it. At one place, I also dealt with changing barn hours such as barn closed all holidays. One change I was not informed about I drove 40 mins and couldn't ride one day because someone was showing their horse to a perspective buyer. I stood out in the cold for 45 mins. With my horse and then it was getting dark so I had to unsaddle and leave. Wouldn't have cared so much but I told her I was coming. I had constant phone calls and texts from the barn owner that was constantly changing her mind and decided she wasn't going to board 2 weeks after I moved my horse there leaving me in a bad situation. That same barn owner asked me for damages when I left due to my horse cribbing and the area that was damaged my horse didn't even have access too. Not to mention other people watching me ride and critiqueing me when I didn't ask for it, if I wanted to be judged I would go to the shows at least I could win ribbons there right? Oh yeah and then there was the great idea of let's get a bull and I get to go get my horse out of the pasture when that big scary beast is in there! Eek! Sorry I'm ranting, but my experiance with boarding has been a nightmare up until 2 years ago. Thank God I've finally found a good place!