*inhales* THANK YOU FOR PLAYING HEALER. OUR QUEUE TIMES GO UP BECAUSE OF YOU!

In all seriousness, it sounds like you had a bad tank. Tanks/healers have it rough early on so don't sweat it too much. Really, I hated tanking until Lv60.

DPS get better at dealing and avoiding damage--they're not responsible for DPS-based raid wipes until, well, just about raids. Tanks get better at holding aggro--they're more responsible for controlling party damage, but focused DPS and strong heals can remove much of the responsibility.

As a healer, you get better at keeping people alive--but you're always responsible for the raid. Lv15 onward. As someone who's gone DPS->Tank->Healer, I think your role is the hardest.

Keep trying and don't let ragers get you down! You'll build muscle memory and sometimes lose it when you try something new. It happens; it's OK! You'll build up better habits and eventually even better memory, and soon enough you'll be a natural at it.