Well its "necessary"ness lies on your judgement my friend,
Has your anxiety given yourself any reasons as to why you need to tell her?
I like to lie on the personal rule of "Less is more". On certain things that have happened in the past... especially involving growing up I don't necessarily tend to express those kinds of things with others... not even with someone as close to a mother or a father. It really isn't much of touching ground for conversation if it is not regarding something that has happened currently.
I noticed you put this as a "confession" to her but what are you to confess? Everybody develops in their own way. I had known a LOT of sexually active youngsters that I was surrounded by when I before high school and people do what they shouldn't do but it's not to really be looked at as a "crime" so to speak. It might be that you feel some sort of guilt or something when you really shouldn't... I mean you were kids. You grow up. There is nothing to judge you on that, thus it's not something really worth expressing to someone else as others may take it shallowly.
If you do decide to speak with her, like I said before... just make sure the time is right and the conversation is headed in the direction of growing up and youth. Not that what you're talking about is inappropriate, but to make sure she is understanding where you're coming from instead of personally feeling awkward or worried. That way you don't come off wrong.