Complex sentence: Lovecraft believed in a purposeless, mechanical, and uncaring universe that human beings, with their limited faculties, could never fully understand, and the cognitive dissonance caused by this leading to insanity.
Split into simpler sentences: Lovecraft believed in a purposeless, mechanical, and uncaring universe. Human beings, with their limited faculties, could never fully understand this universe, and the cognitive dissonance caused by limitation leads to insanity.