I don't think I know the whole story here. I have read some of it from another thread, so if I'm way out of whack, I apologize in advance.
Sounds to me like you have more than just a love of horses going on; a business as well, or at the very least a hobby that supports itself.
The facts of business are: that you will always have somebody that badmouths you and you will always find people that believe the bigmouth. You will also have clients that love you, clients that can see through the gossip and clients that need some time to sort out in their heads what really has gone on and who to trust. This will not correct itself in a "few days" as you say.
The facts of competition are: some business people can sleep at night no matter what they do and they will make a living. Some people need to follow a good ethic to be able to sleep at night; they will make a living too, but the rewards may not be as high financially in the short term. Which one are you? You need to know yourself, so you know what kind of business decisions you make.
The customer that did call you and was cautious? Well, she DID call you, right? I mean, RIGHT? That was your opportunity to be sane and solid and offer your very best promotional campaign for YOUR services and why YOU are good. Never mind being vengeful on the gossiper; that would not get you anywhere. I actually just had a client today tell me that she absolutely would not work with my predescessor and she said, "I'm sure I'm not the only client that feels that way." It was an open door invitation for me to badmouth my predescessor, but I didn't. This client asked a couple more times how many other people don't like her, but I refused to comment, though I absolutely agree and there are many others that feel the same way. If my client is happy with me, that is what counts. I don't want to be chosen because "the other guy is no good." I want to be chosen because I AM good.
So, if you are done with horses, fine. Doesn't mean it has to be forever. If you are done with training, fine. But look closely at your reasoning and decision you have made, will make and want to make. Will this same thing happen again in another business? Absolutely. You can't be a businessman if you can't take the lows with the highs. It's the game.
Good luck with whatever you decide.