you dont say what kind of breed whinney is but 6 is still young in anyones book, particularly some of the bigger draft / warmblood type horses who can take until 8 or 9 to get a real maturity about them.
First off rule out pain......... check your tack, have a check over your mare for any tension spots / soreness - even better get a bodyworker out if finances are up to it
Once the pain factor has been ruled out you can move onto schooling / behavioural issues.
Often a common cause of rushing is imbalance of the horse and / or lack of confidence. A good way of solving both is to slow everything down and get her to pay attention to YOU!
On the ground I would do simple obedience work, preferably in a cavesson and lunge line (you in hat and gloves) and work on leading with her walking very slowly by your side - whenever she starts to pull, make her halt and then start again. Everytime she walks nicely and halts nicely make a fuss of her - I advocate clicker training, its a brilliant way of communicating exactly what you want in a positive way.
Set yourself up to succeed - just walk a few steps at a time so she doesnt get chance to "tram on" and then halt and praise. Then you can start doing some lateral stepping work on a circle like a leg yeild on a circle
this will get her thinking about how she places her legs, help co-ordination and balance and it will start to strengthen her HQ's which will help when ridden. It will also help her understand the leg aids when you get back on in that pressure against her side doesnt mean GO GO GO!! lol!
As her attention and obedience improves you can let her out on small circles while you walk a circle to - this means the circles are not to small but she also isnt to far away from you to loose her attention. Get her used to walk / halt commands and "steady" again to be transfered to ridden. When she is understanding all this and is calm and attentive you can do the same kind of work in trot - and then in canter. but start off with short bursts so again she doesnt get chance to get onward bound.
When you get back on her, you can do the same kind of exercises as you did in hand - the lateral stepping, the walking / halting so she is attentive and really listening to your aids and not running through you.
If you get your communication right it wont take long before she really listens to you and if you really praise the good stuff she will really want to get it right! Of course this may take a few attempts for it to sink in as a habit..... so dont get frustrated, do little and often, if she barges through your aids, turn a small circle and ask again. Dont get into an argument, be calm and she will also become calm once she understands.
It will take quite a bit of effort from your part at first, she might be a little reluctant to relinquish "boss status"
but if you can persevere it will be more than worth it
