Tonia,
I am sorry your witnessed this horrific accident. I think TIME will
help you to get over it. But since you confess you have worried
excessively about loved ones for years, the only way to undo that
thinking is with cognitive behavioral therapy that you get with a
psychologist. You learn how to undo the negative thinking (excessive
worrying) and redo your thinking more realistically and positively.
Give yourself some time with the visions about the accident. That
is normal since you were there. As far as the pattern of always
thinking like that, you can get help. Just have your parents call
your family physician for a referral to a qualified licensed psychologist
who speciallizes in cognitve behavioral therapy.
Good luck to you. I have posted to a site about excessive worry.
Limo