Take heed, going from riding regularly to just doing walking is not going to benefit this horse either. Do lots of transitions. Walk trot, trot walk halt, backup walk, walk trot, trot canter, canter trot walk, trot, canter, walk, etc.
As soon as she is nice and easy going let her continue and then transition again. It'll help to rate her speed so she isn't strung out, so long as you transition properly.
She sounds like she lacks both training and balance. Balance can be improved by trotting. Lots and lots of metronome trotting. Like others said, you have to dictate the tempo to the horse with your posting. Pick a tempo and trot to it. Be patient and consistent until the horse figures out the answer is to come back to your selected tempo and not her own. Provide her with generous, but consistent contact. You want as long as a rein as you can get away with to let her stretch into it, but not so loose so that you cannot feel her on the other end. Once you can set your watch to her trot, then you can start to work in some canter. When you get to that part, keep her on a circle and only canter as long as she can stay balanced, then come back to trot. Hope this helps.
Thank you so much everyone! Today I lunged her first then did a lot of walking, some trotting but brought her back to walk when she sped up, did some backing up and standing. Used an english saddle too, she was much more comfortable in it and can actually feel the contact. I think I'll continue to use english instead it's much more comfortable on her even though I usually like western better. Working on slowing my posting as well. She was so much better. Trying to use every one's advice and will stick to it!