It seems that if I have a tank that knows how to hold threat then it's easy but once all the DPS is pulling aggro I can't keep up. there is just too much going on all at once and my brain goes dead.
Any tips on this would be great even though I have the feeling this is just learn by experience.
Essentially you're right, what you're describing is a common phenomena even for experienced healers, though as we heal more our threshold for freezing in horror and dismay rises. So when everything goes to shizzle in a heroic I generally keep healing through and survive, but my first few times on firefighter or P3 anub heroic, I definitely wasn't performing at 100% because my brain was screaming 'OH SH*T' so loud
Healing is about making continuous decisions, deciding who is the most critical healing target and what resource of time, mana & cooldowns to devote to him or her. The more you face those decisions the smoother you become about making them, till eventually it's instinct - and a situation has to be truly horrendous to cause you to panic or freeze.
As Disc you have a bunch of advantages if you find yourself in a horrible group with threat everywhere:
Pre-shield the aggro monkeys: Shielding the aggro-monkeys buys the tank time to taunt, and you time to start spamming on them, or time to crank up Prayer of healing, or on a spectacularly bad pull time to decide to blow Divine Hymn. On a druid or holy priest you can pre-hot the aggro monkeys, but shielding is far far better.
Penance: Brings up a single dps fast, or if everybody is taking damage, allows you to use PoH and then bring the tank up fast. Holy has superior AoE tools, but if the tank is poor often doesn't have time to use them - disc can make time.
Pain Suppression: You can throw the tank a PS and a heal/PW:S and pretty much leave him be for a few secs while you stabilize the party with PoH spam. If things are bad he hopefully used a CD of his own and the combination will make him near invulnerable for a while.
Power Infusion: If it's very bad you can blow this for super fast PoH spam. Combined with shielding on the lowest party members, it's extremely powerful.
Finally the most important tip of all: it's fine to let dps die at times. If the tank is bad, or the dps is over-aggroing, either way the dps need to take more care. I will generally pick one dps who is my guy, who has enough power to pull us through the fight, and I'll make sure he survives. If the blue-geared mage pulls threat and the imba pally pulls threat at the same time, and my tank is so far off the crit cap I could cry - then sorry mr mage, you may be taking a dirt nap.
True. As disc, your healing pattern optimally should look like this: PWS, some other spell, PWS, some other spell, ... or just PWS, PWS, PWS, PWS
I really wouldn't recommend shielding anything and everything when you first start healing especially. In most 5 mans, only the tank will take damage for a significant portion of the fight. Keep a shield (and ProM) up on the tank at all times, and maybe even yourself just in case. Otherwise, it costs a lot of mana to spam shields for pretty much no reason.
You aren't a new player, you know the dungeons - only shield the whole group when there is going to be party wide incoming damage. Shield dps if they take damage or aggro to give you a buffer before you heal them up or heal the group.
If you already shielded the entire group before anything even happened, you will be missing out on the borrowed time haste buff if you need to spam a lot of actual heals. For example at the last boss in trial of the champion, I rotate shield/PoH/shield/PoH - if I'd already shielded everyone I'd only have one hasted PoH until the weakened soul ran out on someone.
Personally I use power infusion on the best caster dps for big aoe pulls or bossfights (usually both with the low cooldown), I pretty much never use it for my own throughput - although as you gear up it may be useful if you're healing something tricky,
I honestly spend a lot of my time in random heroics trying to beat the worst dps we have. Holy is leet skeet, leave disc for pvp. My opinion of course.
Also wanted to add to the discussion that I gemmed and enchanted maximum spell power on everything for heroics (all +23 sp gems) because the fights generally last about 1 minute, maybe 2 with bad/dead dps, and even with constant spamming and no mp5 it's almost impossible to run out of mana.