The first assertion involves the acknowledgement that regardless of how wise, prepared, or strong we might think we are, reality is always able to inmerse us in unexpected (or simply new) combinations of conditions. For instance, you become a father/mother... meaning it all, that is, you get the news of the pregnancy, you go through it until birth, you meet your helpless child, you acknowledge a conexion to this new sentient being (no longer an abstraction as a fetus), and, never last, as the infant grows into a child and later into young adult, your relationship with him/her evolves and so do you as a father/mother. The circumstances brought about by becoming a parent are pervasive, deeply impacting, and no amount of wisdom, preparation, or strength can prepare you for them.
Parenthood is an extreme example (as a circumstance) because it is a life-long experience. Most circumstances, by far, are brief in time. In fact, even parenthood can easily be divided into small time-constrained segments... and this is more so as you have your second, third, fourth child.
A circumstance can be seconds long as, for example, when suddenly another driver jumps a red light and you instinctively hit the brakes to save your life (and that of others). A circumstance can be minutes long as, for example, when you are being fired or, worse, when you are firing someone. A circumstance can be hours long as, for example, when you are trapped in an elevator.
To sum it up, the first issue is that circumstances can and do inform/supersede/overwhelm the individual. No point in making yourself believe you can be spared from their occurrence and their power over you.
The second issue had to do with time limits. As explained, circumstances are, by far, time limited. Yet, depending on the set of circumstances and an individual's disposition, circumstances lose their time limit. This is of crucial importance to psychological health.
I will give a real-life example that illustrates this. Some 15 years ago, there was an earthquake in Kobe, Japan. More than 6,000 people died as 200,000 buildings collapsed (during primary effects). A friend of mine was sleeping when just before 6 in the morning his three story aparment building became a pile of rubble. He woke up falling from the top floor together with the entire building. The earthquake was some 20 seconds long but the settling of rubbler took longer. When it all came to a relative stop, he checked his body and realized that, despite the darkness, he could barely see. He searched around and saw a point of light. He crawled towards it, losing what was left of his shredded clothes and eventually emerged on the very tip of a heap that was his building. He looked around and, where once there was a city and one couldn't see past the next building, there were now as many piles of rubble. He sat there, naked, for an indeterminate amount of time until a rescue team lifted him up to a hospital.
My friend is not a pain in the arse about it, but he tells the "story" often and to whoever will listen. He has done so for 15 years. I remember when he told it to me the first time as I went to pick him up (housing displaced people by relative/friends, etc). I remember him retelling the story to my wife and to many others on the following days. Gradually, however, he came out of the shock and started to include words to the effect of "I think the experience traumatized me". There were other gradual modifications to his tale, small changes that over time have snowballed. We moved away from each other a few years back due professional responsibilities but we still see each other several times a year. Nearly every time, he tells me his story. It no longer corresponds to the original except in the broad strokes.
What has happened? My friend suffers from a very common malady. A set of circumstances that happened once, keep happening in his mind. In a sense, he does not realize the earthquake has passed. His mind cannot let go of the event, the circumstances, and the endless replay makes the memory gradually accrete with small "errors" or "noise". The result is that, today, he remains traumatized but not by what happened 15 years ago but by what his mind keeps on reconstructing afresh today.
The example is extreme... but many of life circumstances are. Nonetheless, we can see the same mind inertia at the level of the miniscule in truly irrelevant events that nonetheless we seem not to be able to let go of. For example, an insult one received long ago, making a fool of oneself, and similar other circumstances that upset the ego. Physical experiences too, like breaking a leg or an arm while doing a particular activity. And, of course, purely empathetic experiences such as those when the circumstance occur to others but we relate so strongly that our mind makes them ours.
The second issue, then, is that the mind makes it possible for circumstances to live beyond their expiration date. Moreover, the mind toys with the actual circumstances so that over time they become a complete fiction with little or no resemblance to the original event. Yes. You read right. My friend is no longer traumatized by the earthquake 15 years ago. In fact, he does not even recollect it properly. He is traumatized by the "version" of those events that his mind has gradually ensembled, bring in other aspects of his life, be those his fear of darkness, of loneliness, of separation anxiety, of losing control, etc. It doesn't matter. None of those things need to correspond to what he experienced. In fact, I can correlate modifications of his recall of the events based on his current circumstances (breaking up with a girlfriend, getting a new job, having just returned from visiting his family overseas, being promoted, etc).
The lesson, then, is to let circumstances go. They happen. They supersede you at times. They overwhelm you too, regardless. But they end. Let them end. Let them be what they are and no more. Don't allow them to entrench themselves into your mind. Let go of them as soon as possible. With practice, you will be free of the kidnapping power of circumstances within minutes, even seconds, of their end. The sooner, the better.
And when you free your mind from the circumstance, you have a better chance of observing it objectively. You can better asses it. You can better understand it. You can better appreciate the extent to which it really mattered. You can better live the rest of your circumstances...