I understand how you feel. I started a century 2 years ago, where 2 women got killed. We were just far enough back, that we didn't see it happen, but we knew something bad had happened. We had to re-route ourselves and at the 40 mile mark, I let my attention wander a bit and hit DH, which ended our ride. We learned about what had happened in the support van, that took us and our damaged bikes back. I saw those women in front of me, picking up our packets. It is quite likely that we would have been victims or very near the crash, if DH had not stopped to pee at mile 15.
I was really shaken up and had some acute stress disorder symptoms for a couple of days. I have not done a big organized ride since. It didn't stop me from riding my normal routes with my friends, alone, or leading groups (small), but the incident totally made me lose my desire for large events.