Well lifebloom is like my main heal and I always stack 3 times so doubling it will reduce mana quite a bit. What instance was it you done that was hard. We done old kingdom the other night on heroic and it was a bastard, complete wipe fest, although me and the tank managed to solo the last boss which made it worth while.
It's not really about how much mana you have anyway. I would say as long as you have about 600 spirit, at least 1.2k spell power and something around 12k mana unbuffed you should be fine. See if you can get some blues from the auction house. I got some stuff from and it only cost around 40g at most for it. A guild that will help you gear also really helps.