The traumatic event causes the nervous system to up-regulate, setting off the stress response - the body's way of keeping you safe. It is natural to experience the stress response, but it was designed to be present relatively infrequently and for only brief periods of time.
Trauma happens when the body doesn't down-regulate or settle and rest again and stays in a heightened state because we continue to feel unsafe. It is like part of our brain and nervous system gets frozen in that heightened state as we try to process what's happening or has happened to us. In essence, you continue to experience that traumatic event, even if it is "over".
As time passes, we continue to function in everyday life, in some capacity, but our subconscious mind is still grappling with the event and what it means to us. It typically stays in our subconscious (below our level of awareness) until something in our daily life triggers it into conscious awareness. This trigger is a stimulus that is in some way similar to that traumatic event - a smell, sound, feeling, taste, sight - that makes us feel like we are right back in that experience as if it is happening right now. Sometimes we consciously recall the specific event, other times we don't recall it specifically but we respond to the present situation as if we are in the past.
For example, John was 10 years old when he stood at the front of his classroom and gave a presentation to his fellow classmates. He saw all those faces, their eyes on him. He was nervous and had butterflies in his stomach. He stumbled over many words and lost his train of thought more than once. The other students snickered and laughed at him. Finally, John was done and sat down, feeling like a loser, humiliated and outcast. Years later, 30-year-old John is at his corporate job. He has been asked to present his project to his team. John stands at the front of the room, sees their faces and their eyes and feels butterflies in his stomach. He feels like a loser, humiliated and outcast and he hasn't even started talking and there has been no sound from his teammates. You can probably imagine how that presentation went!
Staying safe is part of the role of the nervous system, and it does a really good job at keeping you safe. It doesn't matter how many years or decades ago the event took place, the body and nervous system remember the circumstances of the events that weren't safe in the past and tries to help you stay safe in the present by causing you to, whether you're aware of it or not, avoid similar events and/or activate defence mechanisms that helped you in those previous situations, such as withdrawing, shutting down, hiding your emotions, getting angry, or fighting.
This can lead to developing unwanted or "bad" behaviour patterns. And willpower isn't strong enough to break this behaviour habit because it is driven by stress and emotions not by a lack of motivation.
There is hope and a way out, but first let's look at PTSD.