Don't break up because someone better might be out there. You could spend your whole life chasing that star. If you break up...break up because your life with her now is not how you want to spend the rest of your days. If that's the case, you owe it to yourself as much as you owe it to her. BTW...does she have any idea you're having these doubts? If not, you need to discuss it. If you don't feel comfortable discussing marriage with someone you're planning to marry in 8 months...what're you going to do when the hard stuff hits?

I've always said that sex is the barometer of a relationship. If there are problems with the relationship that leaves one of you feeling unhappy with the other, odds are they won't be overly enthusiastic about bedding them. Is she happy with the sex? The frequency & position might just be what she's comfortable with. Again...conversations and compromises might be in order.

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Though I am different from you,
We were born involved in one another.

For it was not into my ear you whispered, but into my heart. It was not my lips you kissed, but my soul.

Complete surrender should not just come at moments in which one faces overwhelming odds, but in the calm when it seems one is personally in complete control of one's life.