Are you healing a lot or just a little? Having 5 of course is better than 4 people. If you don't feel bad or annoyed, ask for a random person to come. Pulling is important. Pull them to you, so you can get off LB's before they even get to you. And use shocks. They are instant so can't be slowed (interrupted). I use food on everyone to give a tiny boost (stam/spi). I would try the 550 stuff if you can use it. For the groups when there are 4-5 mobs. Yeah, that is annoying. Try to kill the caster first (cause no grounding to soak up casting). I have earthshocks setup for 4 people. Alt 1,3 go first, then 2, then 4, then main. Using:
/castsequence [target=focus-target] reset=7 Earth Shock, , , , , ,
/castsequence [target=focus-target] reset=7 , Earth Shock, , , , ,
/castsequence [target=focus-target] reset=7 , , Earth Shock, , , ,
You should be able to completely shutdown their casting. Just absorb the void, he won't do much. That leaves 2-3 melee. That is where stoneclaws come in handy. I would vote for 3 stoneclaws, 1 stoneskin (add in 1 strength if 5 people). You should be able to kill the caster and then turn to an add before you need to heal at all. I use mobs to decide who needs healing. I use:
/castsequence [target=focus-target, help] reset=combat/12 Lesser Healing Wave, , , , Lesser Healing Wave
/castsequence [target=focus-target-target] reset=combat/12 Lesser Healing Wave, , , , Lesser Healing Wave
It lets me heal whoever I pick but mostly whoever has aggro. The person I pick is incase I heal a person outside of my group (BG). I use that on a staggered setup. Alt 2-3 heal together (has mostly resto talents), 4-5 together (mostly casters), and then main (since usually stunned/sapped/feared/etc). I use lesser healing waves since they are faster and usually prevent overhealing (since smaller).

Don't give up. It takes practice. It isn't all glitz and glamour. But when you realize you don't have to ask for a group, you have no ninja's, no people that leave the party, no CC breaking, etc, you realize you did a good thing. I focus on macros and defense. Other people might be different. Just find a strategy that works for you. Good luck.