Rule number one of getting married/making a marriage work: act like an responsible grownup and COM-MU-NI-CATE!

This means telling what you wrote straight to her. You clearly don't know this girl very well, so at this point you are in love with a idolized version of her. The longer you play this chicken game of avoiding really talking to her (on a deeper emotional level), the longer you are fooling yourself with this idolized version of her.

Tbh with you, are you primarily with a 20yo because deep down you hope that someone at her age doesn't have a "high body count" and you want to leave this fantasy intact? If so, nothing wrong with having insecurities but your GF deserves to know that and decide for herself if she wants to be with a men that equates a womens worth with the amount of sexual partners she has been with....