it sounds like your doing casual content and for healers that comes with a unique difficulty because how much dps you can do depends on how well geared you and your tank our. Since your already max lvl im guessing most dungeons your gear wont be an issue, but your tank could likely be lower geared and you can either check that before the dungeon or just wait to gauge how much damage they take on the first pull and adjust accordingly. If your tank is adequately geared and using cooldowns to mitigate damage feel free to do more damage and if they are undergeared lean towards focusing on healing or suggest doing smaller pulls. at the very least i try and put dots up on all the mobs and (im a white mage) throw out stones and holys between healing. and the most important thing is to not worry about when you make a mistake and to grow off it, if your tank dies because you were pushing your dps too much and you wipe just admit your fault to the group and learn from it, most people in game are a lot nicer than people on the forums give them credit for :P.