Pretty much everything Mister and AmurT said.
In general as a healer, you want to get into the habit of healing only when you NEED to heal, and letting regens tick the party up rather than you wasting a GCD on Helios or such.

Rely heavily on your OGCDs to heal. Earthly Star is great for heavy raid wide damage, and Essential Dignity can get a tank up quick.
Everytime you're not healing, you should have your GCD moving with Malefic III and Combust.

Of course, also keep in mind that AST isn't capable of dishing out quite as much personal damage as a SCH or WHM