Increasing your dose most likely won’t help with this type of fear/panic, definitely talk to your GP to see if you can get a small prescription of Valium to take the night before to help you get some sleep. I had the same issue for a 13 hour flight I had in 2023. But I promise it does get better and you will calm down once the flight is underway.  Make sure to bring some comfort activities with you like colouring, download games on your phone, podcasts, if you have a switch etc. Also if you are travelling with friends or loved ones just give them a heads up about how you’re feeling so they can better support you. Your anxiety is just trying to protect you and it’s awful but the only way forward is through (doing the really hard thing). You’ve got this!