I ruptured my MCL and "partially" tore my ACL playing rugby just about 7 years ago. A few years ago, I had a horse trip while I was riding and in my effort to stay in the saddle I felt an all too familiar pop. I haven't been to my orthopedist since, but the instability I experienced after the riding incident was greater than my rugby injury.
Thankfully being a physical therapist myself, I have managed to keep my knee as strong as possible by maintaining the strength in my knee as well as my hip in order to avoid the surgery bullet. Still, when I ride, I do ok with most riding activities, but when the horse spooks, or bucks, or jumps particularly hard, I can literally feel my femur and tibia subluxate (partially dislocate) and sometimes I wind up with a full out joint effusion (massive swelling). I've had my knee drained and shot up with cortisone on 2 occasions.
anyway, I would recommend you find a physical therapist that specializes in sports medicine. He/she will understand your desire to continue with your sport. Despite the "unplanned" things that occure during riding, I think one of the reasons I have avoided surgery for so long has been because of my riding. The braces my orthopedist gave to me were too bulky and tore th crap out of my saddle, but I wear a standard soft hinged brace when I ride to help keep my knee from going out on me when I ride and it seems to help somewhat. Just be careful when you dismount. That's one of the hardest things for me to do and I often can't put weight on my right leg for a good 5 minutes if I dismount the wrong way.