Complex sentence: His mother eventually brings home a man she loves and plans to marry but Koji has a hard time coming to terms with this because it feels as if his mother is denying the existence of his father and by extension himself.
Split into simpler sentences: His mother eventually brings home a man she loves and plans to marry. But, Koji has a hard time coming to terms with this because it feels as if his mother is denying the existence of his father and by extension himself.