I don't know if their is a way to break her from this other than giving her something to calm her down like Cool and Calm! I use it on my stud and it helps he does the same thing! But if there is a way to break them from this I would like to know as well!

I can think of two ways to curb this dangerous habit, both seem harsh but drastic measures are in need.  Not standing on all four feet during travel can place your horse at risk of injury during fast braking or evasive maneuvers.

Two methods right off the top of my head:

1.  Shock collar, a way to correct her while in transit, operates by remote control but could send her into an explosion that would ruin your trailer.  I'd try this on the ground while being held so she doesn't learn to set back.

2.  Chains.  You can use a kicking chain on the front legs and when the horse paws the chain smacks the front leg.  This can be put around the pastern or up above the knee...I have seen or heard of both methods.  This is immediate correction and the horse trains themselves.

Similar to huntseat's #2 suggestion:  Take a small dog collar that will fit just above the horse's knee, and tie a horse shoe to it so that the shoe hangs at the horse's shin.  When the horse begins to paw, the shoe will bang it's leg and the horse will associate the pawing with the discomfort of the shoe thumping his leg.  This may seem a little drastic, but it has worked for a friend of mine and as huntseat stated, pawing in the trailer is not only annoying, it can be dangerous.         Good luck.

RIDE ON!

     Trailering can be stressful for horses as instinct tells them not to go into a dark enclosed area.  But we get them to do it.  It doesn't mean they are happy while traveling.  It's also theorized that while traveling their stomach acids churn and since horses can't throw up, it irritates their stomach lining as it sits there churning away.

    I used to have a QH that was a nervous horse and after owning him for about a year and camping 1-2 times a month during the season (Oct - April here) he started colicing when camping.  I suggested ulcers to my vet but back the ('96-'00) they had only found them in TB's so he dismissed the idea. I know firmly believe he had them and that same vet now concurs.

    In order to keep that from happening with my current horse, I use a product called Miracle Clay to coat the tummy when trailering.  It's sold by Dynamite and is a powdery product that you mix with water.  I give about 1/3 of a syringe orally before loading and again after unloading as a preventative.  I haven't had any colicing issues with my horse in the 3 years I've owned him.

    It could be that once you start moving, the horse is anticpating the burning sensation and paws to try and get you to let her out...  just a thought.  I have also had success giving the miracle clay to horses that started chewing wood (mine) which is a sign of ulcers.  He stopped and hasn't started again and it's been 2 years since he started the wood chewing.   It's inexpensive, lasts a long time, easy to give, and horses seem to like the taste. 

    Much easier method than those suggested for both you and your horse.