Scar tissue can happen for a variety of reasons - trauma, strokes, surgery... and the brain can work around some of it. Like real estate, it is all about location location location... and of course size is a factor.
There really is not anything you can do except live a healthy life - avoid stroke risk factors for instance. I don't think there is a real way to avoid trauma as well, no one really looks for that unless you are out on a rugby field. It would have to be pretty severe trauma, like what you had before, to *maybe* cause damage.
I hope that you can come to terms with it and learn to live your life.
Apparently you didn't suffer from any symptoms so which means other parts of brain took over. Brain is so unpredictable and some times people with mild concussion suffer from disabling symptoms for long time and some time people with severe brain injury get back to normal life within months. So you are going to be fine and nothing to worry about.
Thanks, I hope you are right! I think my biggest worry right now would be a car wreck, but I guess I can't think about that every time I get in a car..