Things do happen and feelings to get hurt, that is also (my rule anyways), you can never make business with friends. It just does not work, there are to many emotions to keep it business.
Despite what happened, I really don't think you should fall apart and let a single person(which was followed by some new clients) take over your dream and your career. You are in a business that is all word of mouth, alllll of it, and you will always turn out to have people who will not like you-regardless of what you do. That's just the nature of the business.
It sounds like you are a good trainer, or at least a trainer that people in your area want. Stay professional and simply explain to those clients who just left you, what happened. Keep it simple, keep it clean, don't say anything negative about them(because it will show bad character and very unprofessional), and move on. Find a new location to give lessons and train horses, spread the word, put some posters up (maybe some adds), and let this start up again. You will get new clients, you will become a successful trainer and you will not let this take you down so easily. You cannot give up on all of this, if you had that much going on.