To add to CJ82's excellent posts, I'd really focus on getting her using her hind end and doing exercises which encourage balance and bend. Big circles, shallow serpentines, things like that are very good. All while driving her forward into a light and soft contact. Work on your position as well, even more so, since even the most well schooled horses respond to the rider's lack of balance by bracing and stiffening in the ways you have described. When you ask her to bend to the left say, make sure you are not braced on the left rein or blocking her with your right hip or sitting too far back or too far forward. All of these rider errors are really common, usually unintentional, but can confuse a horse and as a result, they brace and act resistant to the aids. Also, do things like turns on the forehand to encourage her to move laterally off your leg. Most horses show resistance behaviours like leaning and bracing and so one when they are out of balance, their rider is out of balance, or both. The horse doesn't know that the best way to move while carrying a rider is by engaging their hind end and being light on the forehand. We have to teach the horse that.
It also doesn't hurt to teach bending and yielding from the ground. Natural horsemanship type ground exercises where you teach the horse to yield different parts of its body from pressure, depending on where you put the pressure, are very useful. Put pressure on the horse's shoulder and it should turn on its haunches. Put pressure at its flank and it should turn on the forehand. Put pressure just behind the girth and it should leg yield away from you. Exercises like that affirm the move away from pressure concept and improve lateral flexibility (if you do them right), sometimes with more clarity than riding if you are a less than perfect rider and can give conflicting signals with your body unintentionally when you're on the horse's back. Also a good diagnostic tool -- if your horse does a lovely leg yield from the ground but does a pish one when ridden, then it understands the aid but might be telling you that you're doing something confusing with your body that mixes up the signals.