I think that horses can carry more than 20% of their own weight. I'm not sure how much more, but they can carry more. But it also depends on the horses conformation. A horse with a long back will have more problems carrying more weight as opposed to a horse with a shorter back. That is why an Arab, even though it's a lighter breed can carry more % of their own weight versus say a TB.
Just take your time and your time and build her up to full work gradually. Riding her daily will help you stay motivated at the gym.