The BO would much rather have you pay your bill in full than end up owning your horse and then try to sell it to recover her money. So the suggestions to talk to her, and work out a reasonable plan to get caught up are excellent ones. In the US, the law that allows a BO to do this is called a "mechanic's lein" and it's enforcable whether it's spelled out in a contract or not. You might want to check out the law where you live.
And I'm sorry this thread hasn't been full of much sympathy or encouragement; but the final advice I'll give is that if you don't have a realistic plan for getting caught up, put the horse up for sale yourself. That's a hard thing to do, I know, but you'll be better off financially if you sell the horse yourself and pay off the BO. Just have a talk with the BO before you advertise the horse and let her know you'll pay her out of the proceeds.