Since you mentioned Brayflox, I have a question, do you use Esuna often to remove the poison? That could be the reason why you run out of mana so fast, The poison hurts a lot more the more it stacks so you end up curing more than you really need to, thus losing all your mana. It's possible that you don't notice the overhealing because the HP is dropping a lot, but you have to make sure the condition is right so you don't have to heal as much.