Left rein troubles

[Debb]

Iv recently backed my mare. We have done plenty of ground work (lunging/long reining) she was spot on.
We have the back lady/saddle fitter/dentist (even though she's bitless)
First few rides she was fine now we have constant trouble on the left rein

She will happily walk round but the minute you ask for trot she starts pulling her head to the right eventually leading img to her facing the fence and becoming impossible to pull back round. She will also attempt bucking if you keep asking. I don't think I'd class it as napping cause she does this at any point you ask for trot and that rein.
Iv also been lunging her still and the left rein has become a problem then aswel. Iv always worked her equally on both reins to prevent her being stronger on one. Someone recommended using a Pessoa and putting the inside rope tighter to bring her in. Iv never used one so unsure if that would work. Any helpful tips?

 

[be positive]

A pessoa will not help with what sounds like a lack of her understanding what is required, I think you probably need to go back to long reining ensuring she is carrying herself straight avoid using the fence so she is working without using the fence for support and reestablish the basic aids to go forward and slow down/ stop.
She sounds to me as if she is rather confused, being bitless may not be helping with this as there is little to support her and if you are stronger on your right side, as many people are, even a little extra weight will encourage her to turn that way so she turns to yield to the pressure and finds that is not required, becomes more confused so tries to put in a buck which makes matters worse.

Backing youngsters seems easy until something goes wrong, they do not read the books, if what you are doing is not working for her you need to go back a step and think about where you go next, thinking outside the box, getting out hacking with another horse, try something different so she has every chance to understand, possibly put a bit in just to make the aids clearer, she has learnt to yield to pressure and is now expected to accept some and go forward into it, using a bit and 2 reins may make everything clearer for her.

 

[Debb]

We did try using just a standard rubber bit which didn't go to plan she planted her feet rearing and bucking (she's far from dangerous hardly comes off the floor) she's the oldest horse none of the other are ridden and the roads near us are deadly so hacking isn't an option unfortunately. We do have a 7acre field to our selfs though. When I rode her a few days ago I asked for small spurts of trot down turning her at the top of the paddock on her left rein and coming down the centre just half way and she was fine. She still veered off towards the right rein