Honestly? I mostly try to stay out of the process on things like this. As they move up and start training the scarier skills (and I think giants qualifies as one of these), I think the mental component is best dealt with like everything else: in the gym, by the coaches.
That said, of course it comes up at home, so I just addressed it like any other life lesson: stay patient, stay focused, keep chipping away at it, the skill will come when it comes so just make sure you're ready... that sort of thing. Sometimes I remind her of past struggles she overcame successfully ("Remember how much you struggled with your cartwheel on beam? You're great at those now!") That sort of thing.
My advice is embrace your role as cheerleader, and leave specific advice to the people whose wisdom and experience you pay for, lol.