The Wave by Algernon Blackwood

Tony had come back. The Play turned very real.

The situation "a trois" thenceforward became, for Tom, an acutely afflicting one. He found no permanent resting-place for heart or mind. He analysed, asked himself questions without end, but a final decisive judgment evaded him. He wrote letters and tore them up again. He hid himself in Assouan with belief for a companion, he came back and found that companion had been but a masquerader -- disbelief. Suspicion grew confirmed into conviction. Vanity persuaded him against the weight of evidence, then left him naked with his facts. He wanted to kill, first others, then himself. He laughed, but the same minute he could have cried. Such complicated tangles of emotion were beyond his solving -- it amazed him; such prolonged and incessant torture, so delicately applied -- he marvelled that a human heart could bear it without breaking.