I think it's important to keep those things in mind:

1) when you have a chance, you should dps..
.. You can train yourself to be more flexible this way (trust me, it helps).

2) understand your team..
.. Not everyone is pro, sometimes.. You have to get ready for the bad situation.
Your job here is to minimize the catastrophe and avoid the wipe. If you get aggro, always go near the tank.
Don't run around tank hoping he get aggro off of you. You have to heal yourself too and every spell has 0.10 sec rule.
you can stop casting spell just before it's about to active and successfully perform running heal.

3) positioning..
.. Always look for the safe place to park in. Get close to a tank, but not too close. Don't over heal the team, remember how much healing you do per cast.

I'm sure you will learn more by yourself. Just remember to challenge your skills, do more than usual. You have the potential to be the best healer in game, just challenge more!