1. You may be as much in the right as it is possible to be, but the customer is the customer. You don't have the right to tell her she's irrational, unprofessional or mediocre. You don't have the right to grab her lapels and shake her into comprehension. You don't have the right to scream and shout and kick her shins, even when she's beating her fists on the floor and throwing tantrums about your being unable to resolve her mistakes, even when she offers no gratitude for the help you offer. Yours but to offer the assistance you can, keep your temper and know when to walk away from games of "who blinks first".
2. Hope dies eternal. Walking away is difficult. You always think, around this next bend will be a solution for us both.
3. Serenity prayer.