I can totally understand your frustration, and especially since anxiety attacks are much less noticeable than a panic attack, I can understand the confusion.
It sounds to me like an anxiety disorder- I have reacted like this often enough, I can relate to her.
But, that's the thing. You're looking at anxiety, not panic. Panic attacks are really easy to see, they're easy to diagnose. Anxiety is not- anxiety causes people to close up, try to hide, and it becomes much, much less simple to diagnose or even notice that there's a problem beyond being bitchy or unhappy.
If she can't take criticism, then go for praise. Instead of "You should try putting your foot here," go for "This move we really have down! Good job!"
A huge part of an anxiety/depression spiral is around negativity, and focusing on it. Anxiety causes a person to worry, to look only at the negative, and to project that negative into the future- to expect it. People with anxiety (myself included) BELIEVE that bad things will always come from the future... no matter what has ACTUALLY come down the line at us. The issue isn't the facts of life, it's what we choose to omit or focus on.
Depression is looking back, and seeing only that negativity.
Like I said, you need to just allow her to be herself. If she gets anxious, she's going to get sullen, upset, short. She's suddenly retreated into her head, and started on the spiral: "What do you think you're doing? You can't do this. You've seen it over and over again- you can't get this. You'll fail. You'll fail, and your partner will be upset with you. They will laugh at you. You'll never get it." As soon as it's any kind of criticism, even if you're as gentle as possible, that voice gets louder. It's a roar in her head that cripples her confidence, her belief that she CAN, her sense of identity. Everything is lost in the end.
Having the studies of psychology is NO substitute for actual experience. I appreciate and respect the outsider's view- I see a counsellor, and I very much appreciate his help in my journey to be rid of my anxiety, but I also know that I am the one leading. He hasn't been through the things I have, he doesn't know what it's like. I must lead the journey.
Telling her "This is just for fun!" may make her think she'll ruin all your fun. Telling her "Don't worry so much!" no matter what it's followed by, will make her worry about how much she worries. Asking her if she needs drugs may make her think she's crazy, needs to be medicated, perhaps she shouldn't live with "normal people". It all depends on her- what her triggers are, how severe she's feeling it that day (or in general), and the sorts of things she's afraid of.
So, as I said: Be with her, let her be herself. We may see anxiety as a bad thing to be battled, but she isn't there yet. One MUST accept and learn to live with these sorts of things before they can be battled effectively. Acceptance for other people can be a huge leap in that goal.
Being positive is always an option- focus on what's good, what's going well, what you're happy with or proud of. She will follow with confidence, she'll come out of her shell if she can, and she will hopefully be able to hear the good things, and not worry so much. It's all about focus.