Personal story:
I met my fiance, who is several years older than me, on a message board where he had something of a reputation for being a player. Before we started talking to each other, I'd seen plenty of pictures of him posing with cute girls at parties and conventions, and, since I didn't think of myself as a "hottie," figured that he would have no interest in me. Most of the women in the pictures were very shapely and dressed in sexy outfits.
When he first approached me and started flirting with me, I was very surprised and figured that it would last just about until he found out what I looked like. Turns out I was wrong.
But it took me quite a while to accept that I didn't have to compete with the girls that he had dated previously. I kept thinking that he was just dating me because he felt like he should grow up and have a "serious" relationship (i.e. one with low physical attraction). It's sometimes difficult to realize how other people see you. Since that time I have come to the shocking realization that lots of people find me attractive, but for a long time I really didn't believe that someone could see me that way. Maybe your girlfriend has similar insecurities?
It was a similar situation to the one you describe, Predicament, in that he had dated quite a few women but had never been in love, and most of the relationships had been very short. He is very confident and if he saw a woman who looked cute, he had no problem asking her out to dinner! Totally unlike me; I've always been one to obsess about someone for weeks before making a move. It was difficult for me to accept at first that his relationship with me was different not because I was less attractive physically than the past girlfriends, but because there were other aspects to the relationship besides physical attraction.
In summation, I don't blame your gf for being nervous about this. While she was out of line to guess your password instead of just talking to you, it is scary for those of us with limited sexual experience to think about our partners being with someone else! She probably does not think of herself as very sexy, or worries that her lack of experience means that she is not as good in bed.
If you really love her, you aren't going to dump her based on the advice of random internet people.
But you should talk to her about her anxieties and what she can do to feel more confident. If I were her, I would most want to hear that you feel lucky to be with a woman like her and that you think about her all the time. Reassure her that you choose her over other women, but let her know that she is hurting both of you by obsessing over your past and that she needs to find the confidence to let it go. Let her know that you will help her with that however you can.
Whatever you do, please don't use the "you are attractive, but your insecurity is unattractive" line. I hate that one because it sends the message that she isn't allowed to talk about her anxieties, which will just make them worse.
Good luck!