You need to play a holypriest for at least ~2 weeks before you get your mana regen ( aka almost none). Before that you will spam youself oom. I took a long break from raiding and when I came back I found I was spamming myself oom in no time, took 2/3weeks to get in the game again. I always keep myself busy and I don't like to just stand around, if the damage is low I don't stand still, I spam heal. Proactive healing isn't that bad anyway with your mastery HoT ticking.