Ok. It was 3 years ago. It was her reaction to a traumatic experience she had. They were mostly drunk encounters, she felt and still feels guilt and remorse about it. She even seems a bit inexperienced. She is a completely different person now.

I think you had a certain perception of who she was (she may have seemed a perfect fit for the woman you've always wanted) and you learned something about her that shattered your perception of who she is/was. It left you surprised and disappointed. You can relate to her because you have gone through a brief period yourself where you did something similar but you can't bare the thought of 16 other men being able to say they had those experiences with her. But I can assure you; (especially if this was 3 years ago, and this woman is dealing with and overcoming her past herself, and is a completely different person now, yet you struggle with coming to terms with this detail about her,) it is something you have to work on within yourself.

I think you are right about it being an insecurity/ immaturity thing or plain and simply, this might just not be the right person for you. There are men who have married pornstars, ex strippers, women with all kinds of pasts who could care less about what others say or their wife's reputation. Also, a woman who has gone through something traumatic and deals with memories of her past, would better fit with a man who can reassure her that her past does not matter. You need to seriously answer if you're that kind of person. If you want to be for her, then just give yourself some time, and let maturity come with age and experience. I am just worried for you that any little thing will happen sometime in the future, for example if you guys meet up with someone she had a fling with and then you wind up feeling uncomfortable and questioning her (to yourself) again... or eventually years down the line you come to realize you guys are not right for each other. However, if this is something you really want to get to the root of and overcome (as you say it has affected to of your past relationships) focus on yourself. Also wisdom comes from learning from the experiences of others. If you don't come upon a man who may be with a woman with a past, who can speak from first person experience, you can always read some books, scour the internet for some life stories, something should make you see that someone having a brief wild moment in their early 20s really doesn't matter in the grand scheme of things, or in the full expanse of a lifetime, if that person has truly changed.