although I would like to second lubylol's comment about it possible being a pain thing, I think it has more to do with her respect with you on her back then anything else.
I know to sit deep in the saddle and give small amounts of pressure but she doesn't react at all to that! And once you get into a lope, there is no stopping her.
I know not to pull back with a lot of pressure, but when she takes off like this, I don't know what else to do.I keep very soft hands most of the time until she gets out of control. I know I should do a one rein stop and turn her, and I do that at the barn.
because of these comments ^^^ it make me believe she knows she can push you around. sounds to me like she is taking riding as just being a game.
Although it is really nice to use soft hands on a horse, it is not always the best/safest thing to do on every horse at every moment.
She sounds like she needs a firm hand under saddle and if you are not willing to get firmer with her at any indication that she is going to act out then you need to find her another home for both of your well beings.
Also I am not a huge fan of one rein stop, although I do turn to it from time to time with green horses I only use it as a last resort.
With my sport and my discipline teaching my horses to one rein stop confuses the heck out of them, because most of the time the same pressures you use to one rein stop a horse are very similar to the pressures you use to turn a barrel.
My mare would probably keep running if I tried a one rein stop on her, that's why to me it is more important to put a nice firm stop (two reins) and a back on my horses then anything else.
She acts like she doesn't have a bit in her mouth at all.
She is in a Medium Port curb bit
Also, why do you use a medium port curb on your horse? Is this what you have always ridden her in? Did her previous owners ride her in this same bit?
Also, you state that she acts as if nothing is in her mouth, what do you mean by this? Does she sniff kneck and stick out her knose when she takes off, or does she just totally ignore the bit when she takes off? Does she ever shake her head from side to side or up and down while she is running or acting up, and I do not mean the antsy I wanna go shakes but the I am not listening to you shakes.
Also, when ridding her does she feel heavy in your hands?
and eats hay and grain.
What hay and feed do you feed your mare? Like lubylol said, it could play a part in why she is acting out.
My recommendation would to switch her to a very well rounded feed but not something overloaded with sugars and corn and all the good stuff :) hahaha My all time favorite base feed is Safe Choice. I have seen it feed to many a horse and all worked feel on it, all of them kept healthy weight and healthy and sound minds, and they were of all different ages. If you are feeding alfalfa switch to coastal bermuda or timothy.
I would work her at least 5 days a week, if you can, in the round pen for a while just working on respecting you. Start by doing round pen work then backing her, but working only in the round pen.
I would also always work her, in the round pen, before you rode any other time as well.
What I think you need to do is go back to ground zero.
Ride her like you would a horse you
are backing for the first time.
(not a horse that is being backed for the first time)
What are the first things you are going to make sure the horse can do?
Me: I am going to make sure they can flex and be as supple in my hands as possible while staying as calm as they can, this does not mean I will not be firm because if you do not show them who is boss then they will show you.
Also, after that I am going to ask them to move forward a few steps and then I am going to stop them and ask them to back a few steps.
I think doing slow work with your mare would help you a lot. Do not even ask for a trot until you can walk any direction in any area with out her getting antsy, just continue to stop her and back her.
When you start to feel comfortable with that, like she is respecting and reacting to everything you are asking her to do then you can move on to the trot. Once you ask for a trot if she starts to act out immediately stop her and back her, walk her a few steps then ask again. You should follow the same routine as you did when walking her, ask for a few strides then stop her and back her, the do it over and over again. Same for once you are ready to ask for a lope. I think with time and work she would be fine.
Also, it would not hurt to play around with some other bits, ask a horse person in your area who may be able to watch you ride her one day and recommend a bit for you to try. I would say while you have her in a safe and confined space, put her in a snaffle.
Also, get everything checked for fitting, you can never be too safe.
I would also get her teeth and back checked, even though I do not believe that is the issue here but I could be wrong.
Sorry for the long post, please don't forget to answer the questions I asked, if you don't mind. It may change something I advised you to do above. Good Luck!