Overthinking is your imagination working for the wrong boss.

The machinery that builds elaborate disasters at one in the morning is the same machinery that writes, plans and invents. It is a strong imagination taking orders from your fear, and the output is detailed, vivid and useless. Nothing is wrong with the faculty; the assignment is wrong.

When you catch it running, give it something else to do rather than trying to shut it down. Have it plan the actual next step, or write, or draw, or work out what you would do if the bad thing did happen. A busy imagination is much less likely to spend the night rehearsing catastrophes.