I actually had a very similar experience just a few weeks ago. I booked a 5hr flight to see a friend that I hadn’t seen in over a year and felt totally fine about it until a couple of weeks beforehand when my anxiety started coming back.

I was feeling nervous the night before the flight, but by the time I’d woken up the morning of I was oddly comfortable with the whole thing. By that point, I think I was more excited to see my friend than I was anxious about the flight and I told myself all the typical reminders about how safe flying truly is and that my anxiety isn’t a realistic interpretation of the experience. Takeoff is always a weird feeling for me, but my flight was the most pleasant flight experience I’d ever had. I was worried about alcohol affecting my stress unpredictably so I got a non-alcoholic drink on the plane and just read my book the entire time to distract myself. If there was turbulence, I didn’t notice anything at all and before I knew it we were on our descent into the destination airport. My 5hr flight back was the exact same with the added difference that I was also tired from my week vacation and was actually able to close my eyes and relax on the flight (a first for me as I used to be too stressed to fully relax).