Agree with NoBaggy. Try first. Take it in advance, at airport. Check you can take a second one when on board if needed.
I have taken Xanax for flying as I was getting sick of the first day of my holiday either being drunk, or hungover as I usually have to get 5 double G&TS before getting on a plane.
Xanax was lovely, I felt calm, dozy but not sleepy, less anxious and although I still hated take off, it took the edge off.
I was really surprised to be walking down the travelator at the destination airport, when all of a sudden I seemed more wide awake, it wore off without drowsiness and quickly.
I can't get Xanax now in the UK, so I am back on the gin. 
Oh and OP, tell the flight attendants you're a nervous flyer, most are very sympathetic. On my last flight in March, one came to tell me that some turbulence was expected, so if the belt light comes on to not panic and the turbulence wasn't expected to be bad. It helped me when the seat belt light came off 5 minutes later.
Good luck. 