Averting A Breakup

My girlfriend and I had a magnificent six-month relationship. But things changed overnight when she took a brief vacation and apparently had some time to think. Now she has withdrawn and 'wants to talk,' an obvious push for a breakup.

I want to marry this girl, but I have never told her so. I think we're a perfect couple, and she makes me happy and (to my knowledge), I did the same for her.

My questions are:
(1) During the breakup talk, should I argue my case, or should I just accept the breakup, figuring that I can make another attempt later?
(2) Are there other strategies for a breakup talk that I should consider? Playing it cool? Being detached myself? Asking to tell her my side in a few days after I've had the chance to think?
(3) If she says there's nothing I did and nothing I can do, do I believe it and accept defeat? I just can't imagine that she has no feelings for me.

I have no reason to think there's someone else. I think we have a big communication problem, and I want to think about the best way to approach it--so I don't blow things. Couples seem to get back together all the time, and I want to be one of them.