If it helps you any, I am the same size as you (a bit taller) and rode a 10.2hh what I assume to be a mini/welsh mare for my BO. Granted, it was never for an extended period of time or very hard (mostly walking and never over a trot unless she bolted). She seemed perfectly fine plodding me around an arena. I'm sure she would have preferred a small kid, but a small kid wouldn't be able to get her ready for small kid riders.
And right now, I am beginning to ride my 12.3hh, 600lb Fjord. I've only been on her twice, but she didn't brace or anything while I got on or while I was on.
Just my two cents. My other mare is 15.2, which is my max for height preference.
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