Holy priest healing has been sucking lately

My priest is almost level 75 now but sadly I have been leveling him as shadow lately and in 90% of cases I'm queuing as dps in the dungeon finder.

Something has happened after I hit Northrend and it's not good. I can't heal anymore.

In Azeroth I had no problems healing. I ran most dungeons as a holy healer and I did great. I loved healing and as a result it was always easy to get into a group.

In Outland I started having some problems. Dungeons were somehow much tougher and I started hesitating more and more as a healer as levels went by. I started running a lot more dungeons as dps rather than healer.

In Northrend disaster struck. I couldn't heal for peanuts. I attempted healing about 3 or 4 dungeons. In one of them I gave up and left the group after telling them that I didn't feel capable of healing the rest of it. In another I barely pulled through to the end. In yet another one I dual-healed with the main healer because he had some difficulty doing it, and still we barely pulled through.

I have no idea what's going on. It could be me, or it could be crappy tanks/groups or it could be the dungeons themselves which are a lot tougher. For this reason I have mostly given up on healing. My leveling consists purely of queuing up as dps in the dungeon finder and questing in the meanwhile.

When healing, I use Healbot and I think it's great. My modus operandi is to put a Prayer of Mending on the tank, then when he takes a few hits I'll put a Renew and then Flash of Light whenever he takes too much damage and Renew can't compensate. If excrement hits the cooling device, I will pop a PW: Shield. That's my main rotation and it worked great until Outland. Back then I didn't even have Prayer of Mending.

Now though, it seems that tanks are taking so much damage that I find myself spamming Flash of Light continuously, barely making it through the fight and finding myself drained of mana at the end. I am aware that Greater Heal is a more efficient spell in terms of mana but the cast time is too long and by the time I finish casting it the tank could be dead!

I didn't mention Circle of Healing but I think it's the best feature of holy healing. Unfortunately it does have a weakness: range. I usually have to stand right next to the tank in order to get everyone in its radius and very often it misses one or two people.

Here's what my spec looks like at level 74: 13/52/0.

I really don't know what to do at this point. Leveling is fine as shadow dps but I really wanted to be a good healer, as this is my first healing character. I'm considering going discipline but I've never healed as disc and I'm afraid I'll mess that up too. In fact discipline has it's own perks in my eyes.

I'll relate a short anecdote about discipline healing. I was running a random heroic with my retribution paladin and at one point after a fight, we were heading towards the next group of mobs when the following conversation occured:
Tank: "Healer don't you need mana?"
I look at the healer's mana bar and it was almost empty, perhaps only 1/5 full. The healer was a priest.
Healer: "No, keep going I'm fine."
Tank: "Oh wait I just realized you're disc. You guys don't need mana."
Healer: "lol"

So to my mind discipline priests seem to be very mana efficient. On the other hand they don't seem to have a cool instant AoE heal spell like Circle of Healing. Generally the disco priests I've run with have performed very well.

One other thing that might be a factor here is that people who are running heroics are often very well equipped and also experienced. In leveling dungeons, people don't have the best gear and sometimes they haven't run that dungeon before. Many tanks are also awful and I've had my full share of crappy tanks, having just today run with a major asshole who insulted everyone, complained that he didn't win a roll for a spellpower cloak and then intentionally pulled a boss and wiped the group.

Therefore, maybe one of the reasons why I can't heal properly is that tanks are very bad and they take a lot of damage. Personally I have pretty good gear, most of it being blue items from dungeons and a few heirlooms.

Maybe this looks like a plea for help but until further notice I think I'll just give up healing and level as dps. Less stress and much less responsibility.