Sadly, when dealing with a person like this, authenticity, honesty, logic and reason take you nowhere and instead just fuel the drama from the other person as they are often already on the defensive-thus their constant drama and vindictiveness.
Forgive yourself in advance and then lie your teeth out, as this person likely is too emotional unstable to be able to actually listen to the truth. In her insecurity, she wants to feel “safe” and that everyone is her friend and if she doesnt feel safe she likely goes on the offensive to attack those who make her feel threatened.
You love her horse, you love her, you are so grateful for the opportunity she gave you and so appreciative of her help and so excited to take the next step as you explore other horses you are interested in. If she insults you, play oblivious, like you didnt understand it was an insult or even agree with her -you are right, your horse was a bit above my skill level now that I think about it. This isnt about you or your skills-its about dealing with her.
If others ask, themselves looking to get juicy gossip to feed the cycle, tell them how amazing she is and how much you loved her horse, how you have loved the barn, blah blah blah, even driving them to agree with you. Dont even give them a chance to create a wedge to have a negative conversation. Then change the subject or say "oh, Ive got to go pee, be back in a few minutes"or “can you help me with this XYZ” ie turn into a half wit who has the attention span of a butterfly.
While the world would be great if we all acted like rational adults, the reality is that a significant portion of the world just never grows up, and you cant make them grow up, only figure out the best way around their nonsense… And change barns too 