We are uncertain of the future and the question of "what if" clings on one's mind.
Why do you think Sunday night is a challenge for so many? Often anxiety originates from fear of what the future will bring as the coming week approaches. Since the future has yet to arrive, this fear is based on doubts.
Why not fantasize a great future, a great week, and start working toward making that happen instead of feeling paralyzed by the fear of "what if" and doubts.