Failing to live well.
Failing to understand mistakes made in the past and determining how to improve into the future.
Failing to take responsibility for ourselves and to turn that responsibility into action.
Failing to take the steps each day towards creating an enjoyable and fulfilled life.
We fail at a lot of things in our lives and from a young age and into our later years but it's not failure we should be most afraid of because each failure is a waypoint on the map that leads towards success and it's the journey from one point to the next that makes up our life's journey and at the end of that journey is the end of all things for each of us; many fear that too bit I don't think that's fear-worthy.
What people should be most afraid of is failing to live life as best they can...and that fear shouldn't stop a person in their tracks, it should inspire and motivate and cause one to not feel content sitting still, staying in place. Never beginning to live as one wishes to live should bring clarity of vision and the impetus to take that fear and turn it to their advantage.