Well that's not your fault at all. tanks should check you to see if u are geared to heal that amount of damage.

Also healing larger pulls requires practice, use Stoneskin before the pull. when tank finish the pull use regen , eye for an eye (if u have it) and spam cure II also if u see tank's life getting really low remember you have Benediction and always remember that Divine Seal and Shroud of the Saints are your best friends to heal a bit more and refresh mana.

Good luck