If you are unable to keep levelling once you reach Lv50, that means you must only have the base game installed and not the expansions. You can keep levelling up to the level cap of the newest expansion you've installed and registered to your account. So if you've bought the expansions and intend to keep playing into them, make sure you install it sooner rather than later. Not only do you have the rest of ARR, but also a long chunk of story at Lv50 before you officially move into Heavensward.

Generally speaking, you can usually level two classes between quests and MSQ even when you don't have the double EXP bonus. You're a long way from the end of the base game if you're in Coerthas looking for the airship, and it probably won't take long at all to get up to story level with another class.