It's not something anyone can do but being on controller I think by playing a summoner for a while will better help you as a healer if targeting is an issue. Also, apparently little known but you can hold L1 and use up and down like you do with party members on controller to target based on list order instead of just scrolling left/right. Perhaps if you mastered this style of targeting it would be super easy to transition between holding L! to DPS and letting go to target party members for healing. I understand it being overwhelming at first and like others have mentioned AST/WHM are alot more heal-heavy than scholar is. Most groups with pretty good DPS won't need both healers contributing much DPS to beat enrage timers so I think after some practice you should try WHM again. That said, good that you found something you enjoy in the meantime.
