I am disappointed to hear about your experience at the store location. I do understand the importance of having good battery life on your device. We are here to help. What is model iPhone this? Did anything change on the device since the draining issue started? Does this only happen with your messaging service that you see the drain?
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Apple HAS released an update that allows you to switch between getting the best performance or conserving power. This is only available when your battery reaches a certain peak capacity.
Over time, batteries degrade. There is nothing you can do about that except for replacing the battery. There was time, I don't know if it is still available, that Apple offered a discounted battery replacement for certain models. If you feel like your phone's battery isn't performing as well as it should and you have taken all steps to correct this on your own, then I suggest visiting an Apple store for assistance.