Ah, willpower! Nearly everyone jumps to the conclusion that difficulty in writing has one root cause: a lack of willpower. It is easier to fall into the trap of blaming oneself than it is to consider another possibility: things are harder to do when you are out of practice. This book makes a strong case for getting into practice by doing something simple: set a timer for 10 minutes and simply write whatever comes into you mind. Do this exercise frequently. It will help you warm up, and it will also increase your ability to focus. It works for me.
Hafer, Gary R. Embracing Writing: Ways to Teach Reluctant Writers in Any Course. Jossey-Bass. 2014. p. 4.