I was diagnosed first with a broken cuneiform bone and after 6 weeks they realised it was still not right so gave me a CT scan and discovered that I had in fact broken 4 bones in the LisFranc region.
Have you had a CT scan? Can you get one done. It is very common that they don't pick these breaks up on xray.
I am 2 1/2 months on and still hobbling around in a Moon boot and, like you, I am a horse rider, busting to get back to riding.