I have been in the exact same situation. I think I may even have sold both of my ponies to the same woman that you are referring to. It was a devastating experience for my family and I often regret it. That being said, it is not a natural feeling to sell a horse or pony.
Just keep thinking of your daughters next pony that will be a better situation for her and thus for you. Increasing her comfort and confidence will make it a much more enjoyable experience for everyone.
Hugs
