OP,
It is very painful to realise a friendship is not what you thought.
You have had excellent advice and you are very self aware too.
You have a good read of her.
She has a lot of surface friendships but trusts you as her confidant.
She will not give that up easily.
You are of use to her and you can definitely expect an increase in pressure.
Definitely have a list of stock replies whilst doing the slow fade.
Treat the interactions as a sport.
You not telling why you are done while she is increasingly pissed off that you won't.
Eventually through sheer frustration she may crack and ask you directly is this to do with the wedding and you can have the supreme pleasure of asking her WHY she would think that....give her nothing, don't say a word, and if she says because you weren't asked you can then deny any annoyance and just keep to the stock replies
There is nothing more annoying to users that to be denied what THEY want.
Don't read her texts for days, bland, vague replies, constantly busy, busy, all the time bright and breezy.
Avoid answering the phone and when you do always be unable to stay on.
You are 100% right depriving her of the oxygen of an explanation.
You don't owe any explanation to someone who has treated you badly, they are not a project for you to fix.
All the while detaching.
Whilst you are feeling pain, I think you will recover quicker than you think.
It's not nice feeling used but she will miss you.
She over estimated her dominance in the friendship, and she will be kicking herself when you will not be swayed.
Be prepared for her landing on your doorstep, as she tries to bully you into getting over this.
"It's not a good time" is all she deserves if you do answer the door.
@Dacquoise you do sound like a lovely friend as do so many others.
I think the point of reprecosity in friendships is such a good one.
It wasn't until I hit my 40's that I did the fade on any friendships that weren't adding joy to my life.
Mutual support is very important and a critical aspect to my friendships as I have aged.