I don't know if there has ever been a more difficult film for me to sit through than this one. Gaspar Noé's Irreversible (2002) gave me both a nightmare and an existential crisis. Viewed in reverse-chronological order, this film will leave you upset with yourself for having watched it and proud that you made it to the very end.
An Experiment with Time: https://en.wikipedia.org/wiki/An_Experiment_with_Time
Article from The Guardian: https://www.theguardian.com/film/2002/aug/02/artsfeatures.festivals
Laws of Thermodynamics: https://en.wikipedia.org/wiki/Laws_of_thermodynamics