I think where she went wrong here was to tell you. In an ideal world being open and transparent would be preferable, but in an ideal world grown ups would be able to handle that kind of info. I kind of get why you can't. I've felt jealous before, of course. So really, since you can't handle that sort of thing the only way for you guys to be together and not be miserable is for her to keep that kind of info to herself. It happened before she knew you, she shouldn't be punished (by having to choose between you and her friends) for not having been faithful then.
see the review