Oh wow, I'm sorry that happened to you. I can't say that it's happened to me...yet, but perhaps it might in the future, I don't know. However, I do walk with a limp, so it's kind of visible.
I can sort of understand why she thought that way, being how many people abuse handicap spaces (like borrow relatives tags when they themselves don't need it, etc.). But on the other hand, if it were me, I would have been upset to be attacked like that, as well.
It took me several years to ask of a handicap placard. I should have asked for one before my surgery, when I really couldn't walk at all. But like you, I was too stubborn. I did not want to face the fact that I was young and couldn't really walk. I did not even use a cane or walker, either when I should have. Everyone told me I should be using assistance, but I just wouldn't give in. Well, I had surgery and was better for several months, but .....I just keep on getting problems and having problems walking, though not as severe as before. I also have severe ankle arthritis, which causes my limp. And so after some long thought, I finally got the courage to ask for a placard, as I've resigned to the fact that it seems my problems just won't go away.
But even now, I don't usually use walking assistance. I always try my best to manage without. But the handicap parking is definitely a plus, especially when I have such problems getting in and out of my car, as the spaces are so much wider.
I am not sure how I would have handled your situation or what I would have said back, if anything. I probably would have said something like...are you aware that young people get disabilities? And for young people, it is not usual to be needing assistance or handicap plates. It's out of the norm. It's can be depressing and we don't want to give in to the fact that we have a disability because it's "not expected of us at our age". I would have asked her if she knows what it's like to be young and have physical disabilities. I don't know what else I'd have said, but probably would start somewhere along those lines, and perhaps give her an education of how young people have disabilities. I wonder if she is aware that little children can suffer the same? (such as juvenille RA or cancer, etc).