Here is what I see. Disregarding the time you classify as volunteering, you are working 3 hours a day, four days a week. Michigan's minimum wage is $7.40, so lets assume that is what you should be paid (I personally think barn labour should be paid higher, but that's just me).
Per week: 3 x 4 x $7.40 = $88.80
Per month: $88.80 x 4 = $355.20
So in essence you should be paying $25 a month for board.
I know you love the work and being around your horse. But if you can get a regular job, work just two seven or eight hour shifts a week, you'll pay for your board, and you can still go out and help out on weekends and you'll get to spend the rest of your barn time with your horse.
I also know how it is when you start to develop a relationship/friendship with your BO, but she is taking unfair advantage of you and you shouldn't have much loyalty to someone like that.