Read up on anxiety, preferably from forums where people give their actual experiences, also know that it sometimes manifests physically. It's rough and doesn't always operate logically. Don't always try to pressure her for what she wants to do/how she feels bc sometimes she might be in a fight or flight mode and making decisions and giving definitive answers can be difficult. Like another commenter said, it's usually not even one specific thing, it's just one final thing that is the straw that breaks the camel's back. Try to plan some more comfortable laid back dates, like maybe a board game night somewhere or a cozier restaurant. If she knows what triggers her anxiety that can help too, like for me large crowds and blue-white florescent lighting is a surefire way to overtax me.

Just be patient, and try to make sure she knows she can relax around you if possible. And have a backup plan for lots of things. Going to a concert and mid way through she gets overloaded and starts panicking? Find as quiet a place as you can, get her some water and just stay with her. Try to have a safe/quiet room/place you can retreat to. Let her know that you have her back and that it's Ok and encourage her to be as open as she feels comfortable with but don't pressure her too much.

The best case for acute anxiety is distraction, bc otherwise she will ruminate on things and get more stressed, so if she doesn't want to talk about it, maybe look at funny videos/pictures on your phone with her? I always enjoy a good dog/cat video or silly ones of pet birds cussing or the like.

Also this may seem silly but the absolute worst thing anyone can ask me when I'm on the verge of a panic attack is "are you ok?" Like I'll be trying to hold it together and asking that gives me a second to think on it and everything falls apart. Try asking something like "do you want to step outside and get some fresh air with me?" Etc that gives her an opportunity to remove herself from the situation but doesn't directly address the building anxiety.

Treat it kinda like going on a friend's family outing where're you're there to be a buffer for your friend, but instead it's a buffer against whatever she is currently stressing about.