Okay, I see your predicament....supposed to go look at engagement rings today! Instead you are planning a face-to-face to end it. Very hard. It's perhaps a cowardly suggestion, but maybe consider calling her and stating you are not up to it and want to give it more thought, this whole matter of engagement and marriage, thus, paving the way gradually for the disolving the relationship. I can see how shifting from picking out rings to breaking up in one day could trigger some drastic behavior on her part. However you choose to do this, she will probably act out, but for today, maybe just time away from her stating you are having to think and not feeling well (which you truly are!)
Seeker