You, sir, are either undergeared, poor at playing your class, or possibly both. In fact, I know you are poor at your class based on the simple fact the OP originally stated Hymn of Hope and Shadow Fiend have been nerfed and are worthless. Lets see, 30% of my mana back is worthless?
I never really chain cast - I spread out the use of Hymn and Fiend if I need them at all.
I then realized that you also have poor gear, because spamming a spell (heal) that costs like, a little over 300 mana, is ooming you. In fact, I am actually gaining mana while casting heal because my mana regen ticks for higher than the cost of the spell while casting.
In essence.... learn2play. I hate to use that overused term, but you clearly do not not understand the mechanics of playing holy. People have said it - stop spamming, watch your overheal. and make sure the raid survives. No need to try and top meters - that is silly and is presently resereved for druids.