Wow, OP, I'm so sorry you're in this place, it sounds unbearably painful.
Would it cost you a lot of money if you cancelled the holiday?
Would your husband be very resistant to the idea of cancelling?
You could talk to your friend to say you're in a really dark place right now and just can't be the fun holiday friend she needs you to be, and ask her to try and understand that. (You don't need to go into all the self-loathing issues - sounds like you have plenty going on in your life as it is to justify needing to stay home.) And if she's not listening to you that tells you something about the friendship.
The reason I'm pushing a bit on whether you can get out of this is that I've been in a similar position, and years later I've looked back on things that I put myself through for the sake of not rocking the boat and thought, "God, I was in such bad shape, I can't believe I put myself through that instead of pulling the ripcord.")
Clearly your husband's infidelity has left a deep scar on you and the marriage. Sounds like you need to address your trauma around that with a therapist. It's hard to imagine how you're every going to feel safe with him again, but I recognise that LTB is a helluva lot harder said than done.
You sound to me as if you're in a full-on depression. Might be worth seeing if that's the case, and also perhaps see if there are any health issues that are dragging you down. Vitamin D deficiency, low thyroid, peri menopause maybe?