Ok, I say go right back down to basics. Work on a lot of transitions and make them perfect.
Ensure that her downwards transitions are smooth and that she doesn't just "fall" from one gait to the next. And make sure she still keeps her impulsion the whole way through. Make your upwards transitions crisp, and make sure she is moving right off your leg as soon as you give the aid.
Then work on all three gaits seperately. Make sure the walk is energetic and purposeful, and don't let her hang her head or get lazy. Make the trot springy and lively - make sure she is tracking up and giving you a proper working trot by getting someone to watch you for 5 minutes and make sure that her hind hooves are stepping in the footprint of her front hooves. Then do some canter work keeping regularity of the strides - either letting her stretch out and loosen her muscles, or keeping her collected and organised.
You could also do some work with circles, serpentines and figure eights, working on her flexion and bend.
Try and practice your half halts more to help with the canter. If she is slowing down when you perform a half halt, make your rein aids smaller (just twiddle the reins between your fingers) and drive her on with your leg. A little tap of the whip might help her while she is learning that a half halt signal is different from a slow down signal.
As for ground manners, you just have to let her know who's boss and be consistent with that. You don't have to be forceful or aggressive, just push her away if she is "nibbling" on you or your clothes until she stops trying, and then give her praise. It takes time and patience, but you have to make sure that you do not let her do that again, or she will not learn, and will just think that sometimes she can get away with it, so she will never stop trying.
Ok I'm going to stop now I think I've said enough