I'm going to be more optimistic than the previous posters. This is a 25 year friendship that has had previous ups and downs. This trip was supposed to be an "up" but your friends life has changed dramatically in the last year. It totally, totally sucks and it is selfish of her, but the fact is a year ago she was in an unhappy marriage and the idea of a fantastic tropical vacation with her best friend was the best thing she could imagine.
Now she is in the giddy honeymoon phase of a new relationship and is thinking about having children. It isn't that you became less important in an absolute sense. She likely still has the same degree of affection and warmth for you as she had a year ago, but she suddenly has a lot of other wonderful stuff in her life right now and you aren't the only light.
She has cancelled the joint party (which is a complete jerk move-I want to make that really, really, really clear. You just don't do that and if you do, it is NOT by a text) but it sounds like the vacation is still on. It would be different. It would be a couples vacation.
I suppose I would look at this and say that it has shown an aspect of her character that is not very flattering. It shows a disrespect for the friendship and a cowardliness of dealing with her changing priorities. However, you have known her for over 20 years and will have to weigh whether this is thoughtlessness and giddy hormones overwhelming a decent person or if this is a lifting of veil to a selfish and shallow person who only wanted friendship when there was nothing "better".