I think the theory works in terms of the everyday irritations we face that are generally fairly minor that people get worked up over. But once something crosses the line from irritation or minor issue into genuine injury or aggrievance, it isn't really helpful or applicable. You would not suggest to somebody who had suffered a physical injury that the pain they're in is a result of their holding onto their own negative perceptions of the situation, rather it would be recognised that those wounds need time to heal, to scab over and then form a scar before anyone would dare suggest that they simply "let it go and move on". Will that scar ever really go away? Maybe, maybe not. Scars fade with time, the pain lessens and eventually the person is able to go about their lives without dwelling on the injury that caused that scar. Occasionally if pressure is applied or they are reminded of that scar, they will remember how they came by it, feel sad or angry and then it will fade back into the background once more.

I find this is more apt a description of healing from mental scars just as much as physical.