The ending of "Misery" is not a happy one, but it is realistic. The poor man can't get anybody to listen to his grief. Nobody cares about him. This seems to be the way of the world. If he were a more important person he could get people to listen to him and people to commiserate. But it would not necessarily be sincere. Virginia Woolf, who committed suicide, was quoted as saying:
The truth is people scarcely care for each other.
In the opening scenes of "The Death of Ivan Ilyich," Leo Tolstoy shows how little his friends, colleagues, and relatives care about the fact that Ivan Ilyich is dead. His colleagues start thinking about what it might mean to them in terms of promotions and salary increases. One of his colleagues has to visit Ilyich's home to pay his respects to Ilyich's wife and to view the body. The scene is almost comical because the colleague is being so phony. He just wants to get out of there so he can go to his club and play bridge.
I wouldn't change a thing about "Misery." It is a sad story--but it is the truth. And there is a beauty in truth, as there is in Jack London's "To Build a Fire," for example. Many modern stories have unhappy or unresolved endings. The stories of Maupassant and Chekhov, for example, never have happy endings like Charles Dickens' "A Christmas Carol" or so many of O. Henry's stories, including "The Gift of the Magi," and "The Last Leaf." A typical Chekhov story ends with the problem unresolved. In "Misery" Iona Potapov tries explaining his feelings to his horse, but the horse cannot really understand. Nevertheless, she gives him some companionship in his misery. I find the ending very touching and the whole story moving because it is so true to the sober facts of life. A least the reader can commiserate with Iona Potapov.
Some people only like "feel good" stories and "feel good" movies. They would not care for Chekhov. They are not interested in reality. They like to get away from reality. I suppose it is just a matter of taste.