Hey Red, I can relate to what you are saying. My H is a paradox. On one hand, you will not meet a more giving person. On the other, he can be ruthlessly selfish. If it is HIS idea to do something he will give til he can't physically give any more. If it is ME asking him for something (specifically social situations like the one you listed) he will bow out with nary an excuse. He even tried to get out of going to my family's Christmas eve celebration! We had quite a spirited discussion over that one.
I wish I had some magical advice for you. I have just had to accept, over the years, that he likes to give in his own way and since he gives a lot I try to overlook this glaring shortcoming.
It is SO hard, though, in the moment when you are faced with someone's utter selfishness (up close and personal) to be the bigger person isn't it.
I toyed with the idea of saying to him "If you continue to disappoint me in the last minute by bowing out of all social obligations that you don't feel like attending (ie, all of em) then you should expect that I will be doing that the next time you request my attendance." However, he has said many times: I wouldn't CARE if you did that!! so I know it wouldn't be an effective deterrant, lol.
I suppose everyone is selfish in their own little quirky ways and this is definitely his area.
My question to you is: Does she have a tendency towards total selfishness or does it revolve only around a few areas?