The essential cause of sadness and frustration boils down to two modes of thought. The first is "I want things to be different" (resisting) and the second is "I want things to stay the same" (Indulging). Strip back almost any thought in the brain to its essence, and it'll be one of those two.

By doing this, we set ourselves up for inevitable sadness. Everything we experience is transient, including the experience itself. The first mode places you out of the moment and into the future, and second into the past. You're attaching yourself to feelings and experiences that don't exist, or wont exist in an unknown amount of time.

Once you start practicing mindfulness you can learn to see your thoughts more clearly. You can catch yourself resisting or indulging certain thought patterns and then let them go without judgement. In essence, your last sentence is spot on. Through reflecting on ourselves in an objective way (as much as that is possible) we can become more aware of who we are and what we do. And with that new awareness becomes inevitable change for the better.