What you are experiencing is 'Catastrophic Thinking' and Separation Anxiety. There are some wonderful CBT tools out there (just look up De-catastrophic thinking worksheets). They basically take you through the process of organizing your thoughts between worst case scenarios, best case scenario, and most likely. Chances are, when you write it down the worst case list is short in comparison. Doing this makes me feel much better but I take it a step further and consider the worst case scenario and make a plan of action should it arise. This not only soothes the fear but actually makes me more confident about facing it. I think observing the likeliest case scenarios might help with the separation anxiety too.