I would definitely find a new barn. If you don't feel like telling your trainer that you found a new barn, maybe just tell her you don't have the finances for taking lessons.
I just recently found myself in a similar situation last summer. I bought my first horse, Image, last March and started shipping him to jumping lessons (he was fully trained in wtc, but never jumped) at the barn that I rode at for 4 years. I didn't know a whole lot about headset and bit accepting at the time, but I knew something was wrong when I couldn't control him and he'd almost smack me in the face because his head was so high in the air. I'd talk to my trainer about it, and she just told me to get a martingale for him (which didn't do anything). And I was basically paying $40 for her to sit there and watch me jump my horse around with 7 other riders in the arena, for 3 months. The BO even tried upping my lesson rates with the boarders (I was keeping Image on my mom's property at the time) and the students who rode lessons horses because hay prices went up! I was already paying extra to buy hay for my horse, why should I have to pay HER more, she's not paying to feed my horse?
I finally had the guts to find a new trainer, who has been working with me ever since to get Image to accept the bit and not stick his little Arab head in the air. I still talk to my previous trainer, and even go ride her horses for her every once in a while. She didn't have a sour attitude, though, like it sounds your trainer does? Regardless, I think it would be wise to find a new trainer; even the fact that you think she is going to badmouth you for going to a different barn is very unprofessional of her. Maybe if she did her job, and you got what you paid for then you wouldn't be leaving.