I'm sorry but all I have to tell u is... live with it... as said before it's a phase all WHM go trough... vl 35 regen will help (but wtill u have to use it wisely... or it will just drain your mana faster than before...) lvl 38 gives u shroud that is a mana regen.... lvl 48 enhanced shroud will make running out of mana your fault (or the dps fault if they take head on every single aoe of the fight...) lvl 50 medica II will alleviate the aoe healing mana cost much more (that skill is creazy good)... and once u start getting lvl 50 equip u wil see your mana pool and mana regen only grow while mana cost of your skill stay put... up to the point where u need to massively use aoe heal/ress in order to have to use shroud on every cd and really spam your aoe heal + ress in order to risck going OOM

also if u'r running with a summoner ask him to do the ress... he have better mana regen than yours (at that lvl) and u don't risk to get a party meber die due to lack of healing while u'r ressing someone else (THM lvl 26 freecast will also solve this promblem but still put a stress on your mana...)