Tanking boils down to knowing all the skills you have, when to use them, what the use them on, do you have to use it later, or so you have to use the skill now. And how good your gear is, is your party enough AoE dmg packed so your CD's last the duration of the pull, do you know all the mechanics of the fights, do you know the spanw places of adds, do you know how to easily/fast grab the aggro, do you know exactly when the adds are coming, are you prepared to it.

if you can keep up with all the things you have to pay attention to and execute them well, then you should keep tanking, or if you feel like you want to be able to do all that. Keep practising. If you doubt yourself you'll just hold yourself back.

So if you really want to be able to tank, go out there and practice, watch videos on YouTube, run content, and keep in mind, it's not always tank's fault. If a blm/brd for example uses all their burst abilities from a range at a add you're supposed to tank, when you haven't even got a first hit on the add, it's their fault for not allowing you to properly get aggro on it before starting with heavy nuking. There really isn't anything that should die so fast there wouldn't be time to wait 3 seconds for the tank to properly aggro the add.