Contemplating the worst... Extreme anxiety.

Hi there, I am new to Stoicism and i have been trying out some excercises as described in Grey Freeman's Practical Stoicism.

I tried the 10 min.. worst that could happen contemplation. it freaked me out. I imagined what would it be like if my 5 year old daughter died, it was terrible! Now I just have a lot of anxiety and what if going through my head. i need some help with this. Any advice?

Corwin