"Not sure why I feel so awkward at the thought of talking but I do. And the more she presses the point, the more awkward I feel."
Because she is violating your boundaries. Listen to your gut.
You also say: 1. you have apologised and she hasn't. 2. that she repeats herself and doesn't let it go
Those are not great signs of a healthy person, OP. There is only one truth, and it is hers. There is only one person that matters, and that is her. She is giving herself the right to 'teach you a lesson'. Not caring, and showing no consideration that she herself is not perfect.
Healthy people are considerate of the other and meet half way. That is why they live in happy relationships, because conflicts get resolved in time and involve BOTH peoples feelings.
I would ask her to write you a letter. She can pour out her feelings on that. [Remember, its her truth and YOU DO NOT have to take it personally or try to defend yourself]. Then, ask her how you can make amends and what SHE would like from you, in order to heal this and move forward.
That pretty much stops the reactivity and takes the wind out of her sails. It is putting the ball back into HER court, and putting her in charge of framing what she would like.
The worst thing to do is stonewall, defend yourself or counter attack. All reactivity creates more reactivity (escalation). Remember, its her truth and that is not your problem. There is a wonderful 12 Step phrase: 'what you think of me is none of my business'. And 'your behaviour does not determine my worth'.
Then, after that, having demonstrated that you care about her feelings and have listened to her sadness (given that space), be firm and tell her its resolved and no more, up to and including cancelling the holiday.
But normally listening without reactivity and asking 'how can I make this better for you?' does the trick. But put it in writing, not in a meeting that can go out of control.