OP, I suggest that you ask the gal who horse didn't toss her head with about it!
She knows the horse, has interacted with her, so is by far in the best position to help you!
If she's unavailable to help for some reason, please get an experienced horseperson to take a look at her in the flesh & to watch you ride her!
Something's making horse unhappy & it needs to be fixed asap. (Usually, with a new rider, it's holding the rein too tight! I'd go down to the buckle with her, in a smaller arena if needed, & just develop your independent seat, starting at a walk. Let her walk where she wants, take your hands off rein & V them over the base of her neck.)
She'll probably still toss her head at first, due to the emotional baggage, but when she no longer feels you on her mouth, she'll stop tossing.
If it happens this way, you'll know what the problem was. Good Luck to both of you!
Last edited by Northern; 02-04-2011 at 12:45 PM.