It's probably one of the more difficult things to do, just like getting into exercising, it's all about structure and staying consistent and being aware of what's coming into your brain.
It's funny you say that, letting their mind master them because in either case it's a product of your mind's device. All negative and positive thoughts begin in the brain. I guess it's just the case of knowing what to filter out and choosing what to believe.
Unfortunately not everyone is surrounded by positivity and therefore sending people into a downward spiral of depression. I have a family member that is struggling with depression and making sure she is surrounded by people with good intentions is extremely important when she has no one. I would keep reminding her to stay positive and filter out such thoughts.
RE: DELETE