She will eventually develop the ability to fly. She needs to develop her muscles as well as having her flight feathers grow in. Her panting is most likely due to her overexerting herself by trying to fly, and maybe a bit of nervousness over it, just as if you tried to run a marathon without the proper conditioning, you could not do it.
I would not let her out of the cage without constant supervision, when you see her beginning to fall offer her a perch to grab onto before she falls to the floor. This will require quick action on your part and may not be easy, will she step up for you onto a perch under other conditions?