Does she stay or does she go?

While the flying anxiety sounds like a totally valid part of this equation to me, keep in mind that maybe she simply no longer wants to go on family vacations. Especially if the slightly older sibling isn't going. Whenever my folks left my brother and I alone when they went away, I never had parties or anything like that, I just enjoyed having the run of the house. When I was that age, I probably wouldn't have joined my folks for a trip if my brother (a year older) wasn't going. You're trying to come up with all these solutions when it sounds like your kid might be telling you she just plain doesn't want to go. Do I force her to go with meds? Can I force her? She is 18 years old, which makes her an adult. You cannot force her to do anything.

Oh god please do not ask her to pay the cost of her unused ticket. As an anxious person I just beg you, please no. Do not teach her that standing up for herself has that kind of cost attached to it.

I can almost guarantee that if she stays home, she is going to be sad and regret it (even if she doesn't verbalize that). When I was 18, I stayed home from a planned and paid for trip to Europe because of anxiety. Now, more than a decade later later, when I think about it, I still feel absolutely nothing other than the overwhelming relief I felt at the time when I was able to pull out of the trip. No regret, no sadness. I have never felt anything other than glad that I was able to stick up for myself. If anyone at the time had told me I was being "selfish" for taking care of my mental health, I can almost guarantee that the relief I feel now would be tempered, not with regret, but with anger at the person who tried to tell me that a vacation was more important than my feelings.

I think I'm just annoyed that she waited until 3 days beforehand to tell me. She waited because she knew you'd be annoyed no matter when she told you.

If you can transfer the ticket to a friend of your 13-y-o that will make the vacation much more fun for him/her. Being there w/o your siblings isn't much fun. If you can't transfer the ticket, swallow the cost and tell your 18-y-o that you love her very much and she never has to keep from telling you anything for fear of your response. Tell her that there is a cost involved, not to make her feel guilty but so that she understands that putting things off due to anxiety makes things much worse than they need to be. Ask her to talk to you sooner rather than later the next time she has something troubling her. My family's situation is somewhat similar to yours, and I sometimes find it difficult to separate my son's anxiety/depression from his age. Telling you at the last minute might be anxiety-related, but it also may be that she operates on teen-time. It's a tough line to navigate, holding them accountable while helping them deal with their issues, and the line moves all the time! But both are important. They do have to be responsible and accountable, and at the same time they have to feel safe and unjudged. Good luck to her and to all of you with her therapy, and be sure to keep up with your own therapy as well. Your kids are looking to you as a model, even when they're legally adults.

Depending on the cost of the trip (and the state of your own finances) I don't think it unreasonable to ask your daughter to pay back part of her unused ticket costs, since she waited so late to tell you she doesn't want to go. I wouldn't make a huge deal of it to make her feel bad about it, though, or use it as punishment--maybe work it out in trade with chores, e.g. "since you'll have the house to yourself, I'd like you to clean this / sort this while I'm away." Then she doesn't have to feel guilty about "ruining" family vacation, because she "paid" for it.

Three things: (1) Speaking as someone who has been around many individuals with panic, I could very conceivably imagine that, with enough pressure, she could keep it together on the plane, and then proceed to have a severe panic attack in private (in some weird sense so as not to inconvenience you). (2) There's a world's difference between forcing someone either financially, emotionally, or both to get on a plane ride and trip (and who knows what else) that gives them palpable anxiety and panic, and the guided approach and exposure to internal & external triggers that would occur in standard CBT therapy for GAD or Panic Disorder or Phobias, etc. I think the important thing is that this young woman wants to get therapy. I don't think there is evidence that the situation is such that the OP is likely to start consciously or unconsciously structuring her life around her daughter's triggers in some sort of pathology-continuing way. (3) I think it's better to assume no malice or deceit on the part of the OP's daughter. The emotional benefit of trusting your daughter's feelings about herself is far better than the victory--or whatever--of catching her in a lie. If she's lying, it's going to be awful whether or not you trust her. Also, bilabial's comment, over and over and over again.

Problem solved this way: she has a CBT therapist appointment next week, is staying home, and 13 year old son found a friend who will join us. Everyone wins.

Your Bahamas family vacation tradition won't last forever. This may be the end of it. Adapt.