This is easy, worry.
I can worry myself into a tail-spin. I've gotten better as I've aged, the last years of college were the worst (that will happen when you are putting yourself through). Lately I've got a good grip on it, I still have my days when it takes over (the mornings I lay in bed alone because V left early to go hunting is the WORST). I know what you are thinking, you have faith in God, what is there to worry about? I know, I know, I'm a work in progress!
For good measure one of my all time favorite verses: Matthew 6:27 "Who of you by worrying can add a single hour to his life?"