sounds like your healing macros might actually need work. Try this approach:

1) Bubble your Paly (focus)

Code:
/cast [target=focus] Power Word:Shield
2) Run your paly into the fray and immediatly do aoe for aggro (make sure you are also using ret aura). Depending on how fast you are damaging you might want to go ahead and instantly put a renew and heal on your main. My healing macro looks like:

Code:
/cast [target=focustargettarget, help] Heal
3) If you feel that you need to sheep you can use a similar code:

Code:
/cast [target=focustargettarget, harm] Sheep
Just watch out for breaking CC, there are far easier and safer macros for sheeping that use focus.

4) After you are in a stable state , start unloading with the dps:

Code:
/cast [target=focustarget,nodead] Fireball
and whatever macro you use for your lock dots.



I think your biggest issue is either your pally is very undergeared, or more work needs to be done with healing.