Well here's a really irrelevant but good question--if your parents are so bent on getting you lessons, why did they get you a horse you physically can't control? Horrible yelling trainer or not, your horse outweighs you, is probably something like a foot taller than you, and could cause some serious damage if he decided he wanted to run you over. But, this might also be your one saving grace. Tell your parents that if you can't find a trainer that you like and who uses the methods you respond to, then you just aren't getting lessons. It may be a bit childish, but explain to them that you aren't about to teach your horse bad manners and lose his respect (which could get you hurt) just because she decided she wanted to be Dozer's bff while yelling at you.
Or you could just wear earplugs and ignore her until she makes a fool of herself by screaming her head off while you calmly walk around the ring. XD