“You would think, though, in light of that she would have maybe scrubbed her horses water trough? Just once? Came out after crap weather and cleaned her own horses’ stall? Came out when it was cold and windy and gave us a hand blanketing or throwing hay? Or even just blanket her own damn horse?”
No! No, no, no, no, no!
You are being too nice again. This must stop at once!
Her cleaning her horse’s trough, stall, or helping throw hay is not enough. The deal was that she would pay you board and take care of her own horse, AS WELL AS help out around the farm on a regular basis equivalent to $200 per month in labor. Her helping out occasionally is not enough. Do not even stray to the mindset of sacrificing your needs to accommodate her manipulation. No, no, no! (Where is my longe whip?)

I wouldn’t get into any arguments with her; just, as others have said, be professional and civil, and state that she owes x board and has not kept to your arrangement for working off board, and I would cc my lawyer (or maybe pretend to, if you I didn’t have a lawyer) on the email and state, “As you will note, I have cc’d my attorney on this correspondence.”