Sounds like the gym instructor lacks communication skills.
I am a martial arts instructor and have big people in my class (and i was very big at one stage) I think the view she might have been coming from was more fear of you hurting yourself, remember each movement or excercise with your extra weight (not being judgemental ) puts so much more pressure on every joint of your body. Remember she will get into trouble if someone is injured in her class due to being excercises beyond their abilities/bodies.
I give different level work to different people in my classes, It allows those that are more comfortable to still move forward and be challenged, and sometimes there will be others who can not do that particular work, Telling someone that it will get easier or telling them whats best for them in her eyes is not always the best way to communicate though
. (though in your case sounds like you could do the work, but maybe shes forgetting not everyone can be painted with the same brush)
Are there other classes with a different instructor, she does sound insensitive to your needs, and with crappy communication and walking out of classes feeling low, its not a great way to keep you motivated.