Are you using cooldowns to avoid damage? are you stunning when appropriate? are you dodging red things on the ground or are you standing in it? are you fighting adds on top of the healer?

In my honest opinion the healer can be one of the most demanding roles in the party.
he has to watch the health of every member in the group as well as his own, struggle to keep up when a member does not comply with one of the above questions.... the list goes on. Demon wall is a great example of this, I have to be constantly moving, meanign I barely get a chance to heal or do anything, top that off when people sand in the purple stuff and red spots. I wish healers didn't get as much hate as they do and people would be much more understanding.