I have two fears that I would say are my greatest fears and it was not until I got older that I realized just how big the fears were though. 

My first greatest fear would have to be losing my family, I am only close to certain people in my family due to issues and we have already lost so many people over the last few years, losing others is a huge fear I have.

Also, another huge fear I have is a fear of dying. This was not a fear I even knew I had until I had kids and it crossed my mind one day and upset me.