I haven't lost a parent, but the feelings would be the worse there is, but eventually I'll move on.
This is a bit random, but I want to say it. In my Dreams for the past year, my Father has only appeared in them two times. The first one was him getting killed. The other one was his Ghost appearing where he died. I remember waking up, crying as I never got to say my final goodbyes and how good of a father he has been to me.
So pretty much my worse fear is not saying the final goodbye.