Sage is now my main, but it took three tries to get there. Twice I picked it up, tried it for a day or two, and quit because I couldn't figure it out. It was only on the third time that it all clicked together. So let me share with you the two most important things that I learned that third time.
First is, you aren't reacting to damage. If you want to react to damage, you need to be playing White Mage or Astro. Most of Sage's tools work best when you apply them before the damage hits. That means you need to learn the rhythm of fights and watch boss cast bars. If you're trying to play catch up, you're going to have a bad time. This playstyle isn't for everyone, so make sure you're comfortable learning to anticipate the big hits.
Second is, don't be stingy with cooldowns. Unless you're downsynched you've got a lot of them, mostly on pretty short recharges. In a dungeon wall-to-wall pull my goal is to layer enough protective oGCDs on the tank so that Kardia heals are enough and I can just spam AoE damage. You start a pull with a big combo and then taper down so you've just got the Kera/Tauro*chole 10% DR by the end. In a Trial or Raid, tanks don't take a lot of damage and your CDs are mostly for patching up people who fail a mechanic. If someone gets a big DoT on them, don't waste the mana on Euk Diagnosis when you can combo Taurochole and Haima then go back to your DPS.
