Its simple, really.

Were you healing properly? you can easily heal 4 adds at the same time, or any boss if that's the case.

NOW, I said that you can heal no problems with all of that, it doesnt mean that your mp will last...if your mp runs out in a boss fight, its mostly the party's fault (unless you were spamming cure II and III, then you have no forgiveness)

Of course, remember that you cant heal spam, this is white mage is different to other cleric classes on other MMOS, like aion for example, where you could just spam heal and buffs, AND DPS, not caring about MP or aggro.

Also, if DPS is dying to AOES, that's also not your fault

All in all, the one that knows if you are doing well or not, its yourself. keep your head up, and keep healing. you will do better and better with each time