Did you take the viagra one hour beforehand ?
I do find that works and then start off quite slow with them, put yourself at ease and it just happens. The worst thing is to overthink the situation and keep thinking "what if i'm doing ti wrong" or "what if she doesn't like it" or "i'll do this in a minute". Just do what makes you happy at that moment in time.
If you still want to try one again to see if it happens again then, i'm unsure who you saw, but go with a bit more mature , more experienced WG and they will know how to handle you.