Don't feel intimidated. Tanking isn't that hard, especially if you know the dungeons in advance, which you do.
Holding hate with Flash/Overpower and combos isn't really complicated. Running out of red AoEs isn't either.
The most complicated part is to understand which mobs will follow you and which mobs won't. Basically the ranged mobs won't, so you bring other mobs to ranged mobs and pack them there.
Mark your targets. Even if you don't know a proper order, mark anything, and you'll learn after time.
Keep you armor up to date, that's more important than on other classes.
Be humble, say it's your first time, say you are sorry when you know you screw up, and don't hesistate to criticize people doing shit... cause there are a lot of people doing shit.
Did they follow order ? Did the pay attention to the situation ? Did they open a pack with a big damage proc ? Are they wearing underlevelled gear ? Are they taking all AoE ?
Remember this game is all about teamplay, so others could be faulty too.
Oh, and level only through dungeons, and do fights like Ifrit/Titan etc in order to train. You'll see how you get better and you'll be more confident.
Also, tank gets instant dungeon pop. Don't hesistate to leave a group full of idiots when you meet one.