I think you might have started to answer your own question. You have nothing to base your thoughts on so you speculate and assume. The "what if" game can lead to a quick downward spiral.

I am facing a medical test and when I learned what it was for, I immediately started to speculate on everything that could go wrong, which lead me to think about things like losing my job, losing my house, dealing with huge medical bills, etc. I was stuck in this downward spiral until my pastor told me to stop and just think about the next step. I know what the next step is - having the test done. He told me I am not allowed to borrow worry by speculating on what might happen if the test results aren't good. I just focus on the next step. For the last two weeks, that has meant taking it hour by hour sometimes.

Someone else mentioned being "present." I like to think about it as "taking care of the task at hand." So if it's time for lunch, I think about and eat lunch. There's nothing wrong with planning, but planning can only be done successfully when it's based on facts and what is actually happening.