Winner winner chicken dinner.
I don't think the OP is being selfish now that she has to share the barn. There was an arrangement put into place that is now not being followed. On top of that the other boarder seems to be lax when it comes to the safety of the animals. Of course one gets a little heated when their animals are put into a potentially dangerous situation. What did you expect the OP to do? Just be a pushover and say, "Oh it's okay if you turn our horses out together for the first time with no one else there even though it could go really bad. It's also okay if you just shut their stall that is to be left open. It's also fine if you change arrangements without letting others (that it impacts) know." I also do feel as though if this happened to a vast majority of individuals on this forum, they'd probably feel the same way.
Either way OP just keep counting down on that calendar! You'll be out soon enough. It's unfortunate it had to end this way.