Body and mind are linked very closely and affect each other. The same holds true about sex. All sexual excitement and associated physical phenomena can disappear in no time if a disturbing thought catches your mind - her loose vagina and associated thoughts, in your case. Add to this a spectator role for the performer - how much hardness, how much erection, plus a worry that time is running out and you have a perfect recipe for a sexual fiasco. Don't take it to heart; everybody, including your studs and casanovas too, is known to fail sometimes. This is not worth fretting over because you always have a next chance. Also remember that sex workers are in this for money and not for you, so no point feeling low. Better to experiment together with a friend.