All riders who care seem to be so hard on themselves. It is a great frustration to me (I do it too, we all do). Have faith is yourself and know that you're moving in a positive direction and that you have guidance from a good dressage trainer to help you. So step one relax and believe in yourself, don't be afraid to be mistakes. Think of them as training opportunities to better yourself. Take every opportunity from the experience you are given. You're very soft and considerate, always good traits and not something everyone has.
First I think it would really help to ride more accurate figures and paying close attention to figures, for example circles or going across the diagonal, etc. On a circle paying attention that you keep a constant bend in your circle shape will help you balance your mare and will help you learn to ride circles and turns more effectively. Riding circles and diagonals and loops it will give you more opportunities to ride them and figure out how to ride the balance. Also getting a consistent feel of both reins, shorten until you feel her mouth (occasionally widen your hands to get the consistent feel) then close them just so you have that connection and feel of her mouth. If a horse braces or gets tight I'll ride the body fo the horse first then I'll lift the rein they're bracing against to get them to soften to the bridle then give the horse that rein with a loop so they take the bit and don't brace against me.
Note the outside rein and outside thigh are for controlling the outside shoulder and therefore steering, so that is something you could play around with. As well as imagining her wrapping around your leg through the turns. Also something to think about and play with when you're in the up position of your post in the trot use your inside leg and send her into the outside rein. Same with canter when your hip slides forward, that is the moment to use leg and half halt. When you capture that moment of the hind leg you will achieve balance.
And please give yourself more credit, you're on the right path.