Teeth!...First and foremost get her teeth checked. Horse tend to lose there baby teeth around this time. A bit might me agitating her mouth. My advice, if it is her teen don't ride her with a bit, maybe just a halter. My filly lost her teeth this fall, and I had to make her food into mash for her to eat it. At first she would take a bit, then stick her nose to the ground and drool it out.
Second, I have trained quite a few colts and never have I ever seen a young horse turn rebellious over night. Animals just don't do that, there is always a reason. My advice, get a new trainer, if he has had colts turn over night, he isn't training them right.
As to the trade, its your choice, do you want to get rid of you filly for the gelding? Will it ultimately be good for you family? I don't know, only you do.
Good luck and God Bless :)