I think you're definitely overtraining. Once a horse knows the pattern there is no reason to continue to keep drilling and drilling. Once per week through the pattern is plenty. Same with the loping endless circles in the arena. Aside from being hard on your horse they can make one arena sour in a hurry.
As far as the bucking goes I don't think it matters why the horse is doing it, you have to get it stopped now. Check the saddle fit and check the horses back and legs but DO NOT allow the horse to buck again. You may want to think about how you bring the horse to a stop at the end of your run. She may not like getting yanked to a stop once you're through the gate. If the bucking persists then I'd get a whip long enough to reach her nose sitting straight up in the saddle and when she put her head down I'd whack her on the nose with it. I wouldn't just tap at it either. Once she brought her head up I'd stop her softly and try it again once everything was calmed down.
I don't allow my horses to buck for any reason. I don't care if it's out of joy or pain or cussedness. Bucking is dangerous and should never be allowed for any reason. If your horse bucks "out of joy" and you fall off and end up crippled it will be little comfort knowing that it was all in fun and the horse wasn't really mean.