Write a suspenseful story about a protagonist who receives a mysterious letter in the mail that hints at a dark secret from their past

Write a suspenseful story about a protagonist who receives a mysterious letter in the mail that hints at a dark secret from their past. As they try to unravel the truth behind the letter, they become increasingly paranoid and fearful of who might be watching them. With each new clue they discover, the tension builds until they finally confront the person responsible for sending the letter and uncover the shocking truth.