I agree that getting some hands on help is the way to go.
It would take a really strong pull from a horse to get the reins out of my hand! Knowing that she is going to pull them out of your hand when you get off, be ready.
Get off, then if you see her start to get really stiff in her body, you know that she is gettting ready to do this. You do it first! You give the reins a good jerk downward and tell her to "smarten up!" or "ah!" anything you wish to say, but get her attention on you. If she pulls back, snap the reins down again. And if you are able to, make her move her hindquarters over sideways.
I think that as a beginner, it's really hard to explain over the internet how to do these things, so , it would be best if you had a more experienced person to help you .