Some days you will not be able to find much to be thankful for. That is not a failure, and it is not a sign of weak faith. Paul, who wrote these words, also wrote about being pressed beyond his own strength.
On the hardest days, lower the bar rather than abandoning the practice. One true sentence is enough. Lord, thank You that You have not let go of me. Say it even flatly, without feeling, and see whether it makes room for a second sentence.
This is not positive thinking, and it is not pretending. It is refusing to let the hardest thing in your life become the only thing in it. Gratitude is small, stubborn and unglamorous. It is also what keeps a person’s eyes up.