Absolutey do not let this trainer undermine your self-confidence or ambitions.
My dd is 18, and in her last year of high school. She too wants to be a trainer, and in fact, has trained every horse she has ever shown. She weighs more than you do, and is working for a BN trainer in our area now. The trainer has her train every young QH that comes in. And these horses are all under 4 years old.
She also placed exceptionally well in her first AQHA show, even though she is overweight.
She rode her 15.2 muscular QH for the last 7 years, and he looks great. If you want to see a video of them doing English and Western, just pm me.
Your chosen profession should make you happy, not your trainer. Good luck and keep your chin up!