Flying a crapload of times and taking longer flights. Sorry, I wish I had a more helpful answer than that, but I don't.

Eventually, after flying a whole bunch of times, I got a little more used to it and would get on a shorter flight completely sober without too much anxiety. I got over it the rest of the way when I started flying internationally and was spending 8+ hours in the air at a time. I think having enough flights where I didn't melt down over not being able to exit the plane got me used to the idea that it was possible, and so then when I would start to panic on each flight I could move past it with just some deep breathing and could distract myself by reading a book or something.