"How do you keep from feeling anger and disappointment?"
You don't!!! LOL!!! The key is to feel them, and let them pass through, not hold onto them. It's one of the biggest misconceptions that being positive means you're not supposed to feel "negative" emotions. Sometimes those negative emotions lead you to important places. If you're in a situation where you're being mistreated, the main way you recognize that is by getting angry. If you didn't get angry, you'd never make a change, and continue to be mistreated.