MP management is the biggest battle for WHMs. Part of the responsibility comes with making the choice of who lives and who dies. It's up to everyone to avoid AoEs when possible, and if DDs are making no effort to do so, you are going to find yourself taxed. As a healer, your main priority is to the tank. That doesn't mean you should just let everyone else die, but what it does mean is that you have to be constantly assessing the situation. If the party is taking too much avoidable damage, or your MP reserves are running low (pot and shroud on cool down), then you might have to let one of the DDs die to alleviate this stress.

Also be sure you're not over healing and wasting MP that way. If a simple regen is enough to keep a party member alive while another is taking some big hits, drop that regen and change focus to the more pressing matter. If your tank isn't taking a ton of damage then you can use regen and just stand there regenerating MP while the fight is calmer.