I think it probably started off as a one or two time thing, but has become more of an issue now you are worrying.
The fact you seem very keen on this lass and find her very sexy, makes it even more likely that you are just finding the situation intimidating as you really want to please her and have a good time.
If you are worried what to say to her, just tell her what you said, that you think she is is the sexiest girl you have ever met, and that you just worry about pleasing her. She will probably be flattered to bits. And you wont have so much worry of her not knowing.
In terms of how to tackle it, it might sound counter intuitive, but to solve the issue, it would probably be a good idea to not push the issue for a while. Sex isnt just about the "act" itself. Try focussing on all the other stuff, and actually make penetrative sex " banned". Do anything but. really just enjoy each other and what you can do for each other, without the pressure of knowing that penetration will come after. If you do that for as long as you bear it... a week, two weeks, you should be so bursting to take it further, that the problem disappears....
(in case it helps, the reason why i suggest this is i had a similar problem with an ex, and that is what was suggested by the expert we went to see).
Good Luck!