I just reassure my gymnast that the reason she is scared (of any skill) is because she hasn't practised it enough yet, and so it doesn't feel secure enough yet for her to feel confident in it. When she has practised it more, and it feels more under control, she will naturally get the confidence in it and be happy doing it. I also remind of her of other skills that she has previously been scared to do, but given time has felt confident enough to do. I think sometimes when they are young they can't look ahead to a time when they won't be scared, so they panic about the fact they are scared now, thinking it will always be like that, rather than realising they just need more time to build more confidence in the skill.