Hey Man... I had a similar thing to this a while ago. I had been in a LTR for around 5 years and not slept with anyone else during that time which affected the old confidenece a little bit!! I got over it by just being honest. She was actually quiet turned on by the fact that she was the first since the LTR and altho the sex wasnt great at the beginnning it definately got better!!

In my experience the key is dont get hung up on Oneitus about this one girl. If you are not massively sexually experienced then dont pretend to be a massive stud or she will see right through it. Be honest about it and also try and sleep with as many girls as you possibly can so that when you do meet girls like the one mentioned in the future you can teach them a thing or two!!!!

Good luck bro