Overwhelm can occur as a result of too much of anything.  From a physical point of view, pain can be overwhelming or heat or cold.  This creates a tangible danger to our bodies.  Another type of overwhelm is due to psychological or emotional pain.  This is usually experienced as anxiety, frustration or anger.  The emotion is real, but quite often the overwhelm is not and may be the result of a habit.  The brain has a way of multiplying the impact of 'too much to do', 'too much to keep track of', 'too many interruptions' and so on.  

The facts are helpful whether the overwhelm is real or made up by the brain.  When there are a few items requiring our effort, writing them down calls the brain's bluff and we can relax.  When there are lots of items, writing them down allows the brain to create some boundaries, to set priorities and to plan.  When there is a plan, the brain now has the needed instructions to proceed, reducing or eliminating the feeling of overwhelm and the resultant emotions. Overwhelm makes us unhappy.

The brain might interpret the additional task of writing down the facts as adding to the feeling of overwhelm and may find all the reasons not to do it.  In this case the brain is an idiot! [Russell]