Many pundits and professionals caution writers to avoid writing novels that don't have a happy ending. Readers will be disappointed, they say. Probably especially so if the author leads them to believe all will end well -- the Fairy Tale ending.
I recently read "The Light Between Oceans" by M.L. Stedman which many may judge to have an unhappy ending. But, the ending is reality...the hero and heroine suffer, undone by what they thought the right thing to do years earlier, which came back to haunt them and alter their lives forever. Stedman sowed the seed for the ending; yet, it still came as a surprise. It was a well-written novel that I encourage all to read.