It sounds to me as though you and your longtime friend have drifted down different paths in life. Sometimes we have friends who fit perfectly into a certain time and place in our lives, but the same friend just doesn't drum to the same beat as we do anymore. When this happens it's good to let the friendship drift off, at least until it might gel again sometime in the future.
You could invest a whole lot more energy and angst in trying to keep things tight between you and your friend, but it sounds to me like she's determined to maintain her current projectory anyway. Do you really want to keep trying to get her straightened out? Is it even your job to do so? It would do her no real favor to put on a fake upbeat exterior and attend her wedding with inner negative feelings.
It might be hard to do, but not doing it will be harder in the long run....if I were you I would thank her and say I can't attend for personal reasons. If she presses you for more info, just say that though you wish her well, you feel you have both sort of gone down different paths. Let the chips fall where they may, at least you will be true to yourself.