Running off, help!

[RubysGold]

So I've had my mare for just under a month. She's been perfect, everything i was looking for in a horse. However, she's now running off with me all the time. I keep her on a farm, so we dont have an arena, just a patch of grass to ride on.
I've been working on fittening her up, but she keeps cantering and sometimes galloping off with me!
There are a few things i think it could be, I recently started stabling her through night as I wanted her mud fever to dry up, also I have started to feed her healthy hoof, but being made for laminitis it has nothing in so therefore shouldnt make her fizzy. Is she just testing me? I changed her bit and took off her martingale bout 2 weeks ago, and she was fine with it, she didnt change.
What can I do to stop her continuing or becoming worse?
Nickie

 

[YorksG]

What did you change the bit from and to, and why did you change it?

 

[RubysGold]

she had a dutch gag, and i just moved the reins up a ring to make it weaker, coz she wasnt strong or anything, but she didnt change until other day
Nickie

 

[scotsmare]

put the bit back the way it was and see if that helps. Sounds like she's decided to take the p*ss a little bit

 

[OrangeEmpire]

Why not try two reins - one on the snaffle ring and one on the bottom so that you can be nice when she is nice and then firm if she tries it on?

She needs to understand that canter is when you want it or not at all. Perhaps you could to lots of trot to canter transitions so she learns that it is on your terms?