I resent that I have to choose between being the bad guy or burying my feelings. She told me she knows it's important to me. I told her it's only important to me if it is to her. Then she said we can celebrate before or after.
She told you that "she knows that it's important to" you, but she is pushing through with the trip anyways because at the end of the day she does not care. You telling her that "it's only important to me if it is to her", is you clearly telling her that if does not matter to her, then there is no point in pretending that your marriage anniversary matters. State this to her using similar words to what I just said so that she cannot later falsely claim that she does not understand this. If she goes anyways, you now know exactly where you stand. It is what it is. That is the condition of your marriage and how she feels about you. You cannot make her care.
All time shares allow you to change dates for a fee. If you do not care if it is the same exact condo (it is shared with dozens of other people anyways), but just a similar one, you will have a lot of date options. Offer to pay the change fee and ask them to change the date accordingly. This offer will make it even clearer that it is important to you enough to pay a fee, and give her the chance to do the right thing. If she does not, then it is on her.
I could not imagine my wife not wanting to be with me on such an important anniversary, but if she did not, I would not want to celebrate this or any other anniversary with her, as anniversaries are only important if both value it. When the anniversary comes around, I would not respond, comment, or like anything posted by anyone (including her) about it on social media, or sent to you directly. Do not let her fake importance by phone or text on that day; have minimum text and no phone contact with her that day, as you should agree with her that it is not important and just another day.