What I tell most of my friends that are leveling up is to get the basics of your class down, and use 1-49 to gain perspective on what you are expected to do, and learn the basic tenants of what your abilities DO. At level 50, the game changes, things that were working before would be totally inconsequential due to new abilities. Almost every class gets game-changing abilities at level 50 that change how you play, or at least mechanically alter your normal strategies.

AK is the first dungeon where the "difficulty" is 10 levels above your character (items are ilvl60), so weaknesses and short cuts you were able to get away with in previous dungeons are now exposed and no longer viable. Ifrit is around the same difficulty, and then you see bigger jumps in difficulty from Ifrit>Garuda>Titan>Coil.

This is a pretty common experience, but I will say that you will probably learn how to REALLY tank in Garuda. In that fight, a lot of damage is being doled out, and you have to worry about positioning a lot. You have a great attitude of getting better with each new experience, though!