As best I can remember, the setting for this one was contemporary Earth, or a human off-world colony with contemporary technology. Whatever the exact location, there are creatures of shadow that present a threat to the world. I can't remember if they are undead creatures, or alien beings, or if their true nature is unknown. I also can't remember exactly what harm they can do to people. What I do remember for certain is that the protagonist is of a particular occupation/class that fights these shadows to protect the rest of society.
The method by which the protagonist and his fellows battle the shadows is something of a psychic duel. The protagonist enters a house, and reaches out mentally against the shadow. The two entities become locked in a mental struggle, with the task of the protagonist to be able to out-darkness the creature of darkness. Something like that. If successful, the shadow is dispelled, and the weary shadow fighter emerges victorious. The cost of victory, however, is quite high. The protagonist talks at length the strain of going into the darkness, and the toll fighting shadows takes on a human being. He and his kind age and become "burned out" far sooner than a human being otherwise would. Eventually, a shadow-fighter becomes too weak to fight on against the darkness. I think the protagonist is nearing the end of his ability to fight, but is not quite finished yet.
About mid-story, the protagonist encounters a man and several followers. They claim to have found a new way to destroy the shadows. The protagonist wearily dismisses the man and his group. For generations, alternative approaches have been proposed and tried. Some come from well-meaning fools, while others are snake-oil propositions from con-men. All, however, have failed. There is only one way to destroy a shadow, and that is the grueling, debilitating method the protagonist uses.
The story's final act comes when the new man and his followers join hands in a circle. I believe they do so around and infested house. They channel positive emotions or something like that. There is a flash of light, and the darkness from this infested house is gone. The new method worked, and it did so without any harm or ill-effect to the man or his followers.
The story closes with the protagonist realizing that his entire life has been spent sacrificing himself, his youth, and his vitality. And now, he and his ways are completely obsolete.
Timeframe of Publication
Not really sure. I'd say 1980's or earlier. I think I read it in a sci-fi mag, rather than an anthology of some sort.