OP very gently stop trying to work out what's she's thinking and why.
I speak as a classic overthinker who has a constant shitty committee in my head going around and around trying to second guess, and third and forth guess other people's motivations and predict their next move.
And blaming myself every time what they did didn't fit with my version of what they "should" be doing.
Sound familiar- it certainly feels familiar from your posts.
So just stop. Stop trying to work out what's going on with her. All you are doing is filling your head with anxiety, what ifs, catastrophising and blame.
You are giving such a lot of head space to something you have absolutely no control over.
And it's doing you no good at all. Just work with the facts, do the minimum admin - here are the dresses, this is the date of the wedding. Are you coming yes/no. I need an answer by Friday or I will assume you answer is no.
Every time you find yourself trying to get into her head and the merry go round starting again remember you can't control people and all the minutes. hours, days s as d nights of worrying don't change a single actual, real thing.
Let it go. This is a time for celebration of a lovely long term partnership. Focus on that. Get you head busy with how grateful you are to have you soon to be DH and your other reliable, caring friend.
I hope you have a wonderful wedding and get her out of your head. She's living there rent free and it's making you miserable.
You do have a choice here. Not about how she behaves:feels/reacts but how you do.
Tell the shitty committee to fuck off. Breathe. Focus on the joy and love you have.