So basically, I'm trying to create dark past/secret for my main character in my first novel, but having some difficulty. This secret has caused much anxiety and depression for teh character's life and I want it to be centered around not something he intentionally did, but was fooled into and wound up hurting people around him, namely his girlfriend (now ex). Basically, he thought he was doing something right, but there was a person (minor antagonist) involved with what he was doing, and they lied to him to take advantage of his kidness/righteousness, which cause the MC to hurt people close to him. Mainly, they were disappointed and upset at him for making such a mistake. As an example, one idea I came up with was that he was heavily into gambling in his past, and gambled away his girlfreind's fortune because the antagonist told him he was set to win a game, but he wound up losing and the antagonist stole the money that the MC lost and disappeared. Any other examples where a person's niceness or gullibility is taken advantage of and hurts others around them?