In all honesty, she sounds childish and selfish. If she cared that much for you, she wouldn't put you through the ringer by seeking experimentation with other men. I entered my own relationship at a younger age than you (I was 19, she was 18). I'm now approaching 30, and we have never once broken up (though there have been one or two close calls). Never once has she said that she feels like she has missed out, because I make her happy (somehow).
Myself, I'd be lying if I said I haven't thought/fantasised/talked about what it might be like to be with other women, but that's all it is: fantasy. And often, reality doesn't live up to fantasy.
I guess my point here is that if she wants to experience 'what she's missing' she's not satisfied with what she's got. I don't mean that as any slight towards you. But it sounds to me like the one thing you need to be is the one thing you can't be - someone else.
If I were in your shoes, I'd put my foot down, say that I want all or nothing, and end it myself. A bird in the hand is worth two in the bush. It's a simple moral, but it fits perfecty here. Letting her go in the hopes she'll come back more appreciative is folly, and will only lead to constant heartache during the interim, and eventual heartbreak when that (likely) doesn't happen. The only thing you can do to keep yourself sane is to cut it off yourself, walk away, and presume that it's over for good, because then you're not setting yourself up for disappointment down the road, and you can begin the healing process right away.
Then, if she does come back, bonus. Just be sure to get her tested if she does.
Edit: Skops has it right.