Sorry for posting this in an old thread, but it's related to the topic, and I figured it would be better to continue here rather than make a new post. I warn you, it's a long story, so read at your own risk!
I'm in a similar situation as the OP, in that I leveled Longtail as a Boomkin since level 40, and have played her as such ever since. I have dual-specced and I can pinch-heal if the healer goes down, but I can literally count on one hand the number of times I went into tree form and actually healed a group. The first was regular Forge of souls, which was surprisingly smooth. I've helped friends do a couple of regular instances where both I and the tank were overgeared for the instance, making healing a minor issue.
However, the night before last I decided to get some practice in. My local friend recently specced his paladin to tank, so I figured we could group and both get a little practice while we got badges. The first dungeon that came up was heroic Forge of Souls. I was a little apprehensive but thought, "Hey, I've already healed regular". That was my first mistake. We hit the first wave of trash, and the warrior basically started doing a whirlwind as soon as we engaged. Suddenly he was the focus of every mob in the group, and I could not keep him up. He dropped as soon as I got off the first HoT. We took the group out sans warrior, who had some snide remark about 'no healz'. I apologised, explaining I was new to healing- and nothing more was said.
We went through the rest of the trash leading up to Bronjahm without too much incident. We ran in, started attacking the Godfather of Souls, and I was healing like a champion- until I suddenly faceplanted unexpectedly. The mage followed suit quickly thereafter. Oops- I forgot to check my own health (something I rarely need to do as Boomkin), and was watching everyone else's. The mage started going on about "WTF, you weren't even under 70% mana, how could you let yourself die". I felt like a heel- I knew it was entirely my fault for not checking my health as well as everyone else's. Of course- in heroic, he does damage to anyone with casters. Stupid of me for forgetting that. I apologised again, and we returned, ran back in and rebuffed.
We finished him off the second time, and I felt okay. Not great- I'd already caused a wipe. However, the second time we cleared him. The mage, for reasons of his own, was whispering me with lots of advice- not something I could read and follow while we were running from trash mob to trash mob. "Always use your global cooldowns, you should never have one free", "keep your rejuvs up on everyone", "you should be using wild growth every time it's on cooldown", etc., etc. I figured perhaps he had a druid healer, so I glanced at his comments and thanked him while trying to keep focused on the fights. We didn't have another incident until we got to one of the ghosts, where I suddenly took a few seconds to cast my insta-cast spells. I warned the group I was getting lag spikes.
We got to the Devourer of Souls, and everything was going smoothly. Then I saw my health go from ~75% to under 25% in a split second. I tried to move and cast a Rejuv at the same time, but before the rejuv even cast, *bam*- dead. The mage started going ballistic, saying I needed to learn my effing class, WTF was my problem, why didn't I Barkskin, etc., etc. If I'd had one iota of warning, I would have. I already felt bad about getting into a heroic I obviously should not have been healing in- not to mention that I'd already caused one wipe. The mage popped some invisible bubble. The rogue said we should try again, but that lovely mage, who obviously knew what I was doing wrong and how "terribad" I was, just wouldn't let up.
I ended up apologising once more for the wipe, and said "I don't think that I can continue given the learning curve, good luck and take care" - or something to that effect. I left group, and my friend basically whispered me with a "WTF are you bailing for", and, given my oversensitive nature, decided that rather than argue with him about going back into instances that night, logged off for the night. Now, I saw it as a double-edged sword. I'd grouped for random heroics but never suspected that the dungeon finder would throw me into heroic FOS. I should have bowed out at the beginning. I also didn't want to bail and leave four people (three, if my friend also left) looking for a healer and possibly a tank. I also wish like hell I wasn't so damned sensitive. It's not something you'd expect out of a 300-lb ex-cop, but I take things very personally. I saw it as a personal failure. I didn't want to play healer again. I play to unwind and relax, and that one incident left me feeling upset and frustrated.
However, after a good sleep and a couple of days to think it over, I realise I can't stop healing just because of one incident. I need practice, and need to study what to do. In the studying aspect, this thread (and the other one linked about rotations) are immensely useful- thanks from another healing newb! However, I also was wondering if anyone has tips or suggestions for a new healer- ideas for where I can safely practice without having it so easy that I don't actually learn how to heal. After all, if I learn when it's sleep-mode easy, then when I hit FoS again, I can see another sad repeat of the story I just shared. So if anyone can give suggestions for how to learn healing after 80 levels of lazerchicken, I'd love you forever or at least I'd be happy that you shared your knowledge.