Sometimes they say yes out of a feeling of guilt or because they feel 'bad' or some other cr@p. I had a girlfriend in my early 20's who left and we tried to remain friends. I was still into her so it became too painful and I told her I couldn't be friends with her.
I'm not implying this is your situation but I have a gut feeling you are going into it in the hopes she will see you and somehow change her mind. Also, tagging along makes it easier for them because it stops them from facing the reality of life without you.
Wonder how she would react if you actually canceled it. I wouldn't do that to get some effect, but make sure your intentions aren't clouded.
MySitch Me-47 STBXW-41 D-5 S-8 ILYBNILWY-01/08 Want a D- 01/09 Physical Sep-01/10 D filed-06/10 Got 50% custody=09/11 Ride that wave!