Sorry you are having these problems!
#1 If you haven't already done it, buy a big tarp to put over your hay. This is a lesson many of us learn, barns are not always water tight...
#2 this may be hard for you, but you need to try to not share so much information with the BO. It should be a business relationship only, and if she is too immature to know how to act, then you must show her the way...
Take her off your facebook immediately. If she questions this, just say you are cutting back.
Do not give reasons to not be at the barn (sick, out of town,ect.). If you are doing self care & can't come out, have a friend or another boarder take care of your horse instead. If she asks you why, tell her "something's come up". No more, no less and walk away. If you are being bothered even while brushing your horse, try putting ear buds in & motioning that you are on the phone. Chatter away to make it look real. Turn your back to her & ignor her attempts to speak.
Eventually even the thickest brained person will get the message that you are too busy to talk.
Oh, do not hand her the month's board in person, mail it. Do everything possible to limit contact & never give her an explaination for anything. If you can't avoid speaking to her, still do not answer her questions, just keep turning the conversation back to her.