Thinking about it some more, @ohwhatisinaname, if you are going to have a conversation with the bride, you should probably be clear about what your boundaries are before you talk to her.
The way I see it is this. Taking just your relationship with the bride, and leaving everyone else out of the equation for a minute, there are two points of view.
Your point of view is that you spent £2500 on attending your friend's hen do in Mexico and she was horrible to you. She might have had her reasons for being upset, but none of those reasons were actually your fault and certainly didn't justify the way she treated you. You are now feeling extremely lukewarm about taking another transatlantic trip in two months' time to attend her wedding, with the same people who behaved horribly at the hen do, without your husband and family, and paying another £2000 for the privilege. If you cancel now you can recoup some of those costs. And you probably won't even get to spend much (if any) time with her at the wedding. But you've been friends for a long time and you're worried that if you don't go it will be the end of your friendship.
Her point of view is that she was looking forward to having an amazing hen do in Mexico, but it was a disappointment, and you left early, which probably felt like the cherry on the cake after several other people bailed at the last minute.
First of all - and this is going to sound rather transactional - you need to work out how much money you can recover by not going to the wedding. Say it's £1000.
Is maintaining this friendship worth £1000 to you?
It's OK if it's not. Friends come and go and maybe this friendship has reached the end of its natural lifespan.
If it's not worth £1000 to you, have that conversation with her but make it clear that you have decided not to attend the wedding. You hope she has a wonderful day and a happy marriage, and hopefully you'll catch up with her next time you're in NYC. Be prepared for this to be the end of your friendship.
If saving the friendship IS worth £1000 to you, it's not as simple as "suck it up, spend the money, go to the wedding to save the friendship" because friendship is a two way thing. If the truth is that she doesn't actually value your friendship, then there is nothing to save and you would be throwing good money after bad.
So if you are potentially willing to spend that £1000 to save the friendship, talk to her, hear her out, but also make it clear how hurt you are that she treated you the way she did after you spent so much money and effort on attending her hen do in Mexico, and that you would like her to acknowledge how much her behaviour has hurt you and apologise. If she does acknowledge her behaviour and apologise, then perhaps she's not an awful person, the friendship may be worth saving, and you may decide it is worth spending the money to still go to the wedding.
But you need to decide what your boundaries are first. There's no point in making her get down on her knees and beg for your forgiveness if you've already decided that it's not worth going to her wedding to save the friendship. If that's the case then just tell her that you've decided it's best you don't go, that you can no longer afford it after picking up the bill for the no show friend, and that you're not really keen on spending another weekend with her other friends. End the conversation by saying you hope that the two of you can still be friends, send her a card before her wedding day, and hope it all comes out in the wash.
And if she doesn't acknowledge her behaviour towards you and apologise, then please don't throw good money after bad and go to the wedding. A real friend would not treat you like this and feel no remorse. If saving your friendship is not worth a simple apology to her, then it certainly shouldn't be worth £1000 to you.