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?
I wouldn't say the gambling example is something he was "fooled" into. C'mon, it's gambling - losing is always a possibility and it's his fault for gambling away his money in the first place. Also, if he lost his money fair and square, how does the antag "steal" the money? MC lost it. No one stole it. That the antag somehow came to acquire the same money afterwards has nothing to do with the fact that the money originally belonged to the MC. Other examples... he hides his ex's essential medication because someone told him she shouldn't be taking it, that she's becoming too dependent on it, and the ex almost dies thanks to this? Maybe girlfriend had some injustice at work or got pregnant and your MC steps in by confronting her boss, resulting in the girl losing her job altogether, and there would never have been a big problem at all if the MC hadn't been dumb about how he handled it? MC signs something, thinking it's something else because he didn't read things properly or because the antag is more legally astute and lied about the meaning of certain clauses in the contract, and by doing so he ends up signing away his girlfriend's house, rendering her homeless?
Your MC could have involved himself with his ex's family affairs. Maybe she doesn't get along with her sister/mother/brother and he approaches that person or sets up a meeting between them. All it does is create more tension between the GF and the family member, then eventually the GF and MC.
His brother, who's always had issues with drugs and the law, shows up all shiny and drug-free from rehab, and asks MC to let him stay with him for just a couple of months until he gets his first paycheck and can afford first and last on his own apartment. MC comes home to find that all of his valuables and his girlfriend's valuables are gone, and over the next few weeks discovers that his brother has charged his and his girlfriend's credit cards up to the limit, drained their bank accounts, and taken out other, fraudulent cards. MC talks his girlfriend out of reporting fraud/pressing charges, so they're actually out many thousands of dollars. For that matter, MC could be the one who committed that set of crimes, in the past. Are you sure you don't want it to be intentional? Doing something genuinely bad, and regretting it, could be more interesting.
The guys say they're from a big TV network and they're performing a stunt on the big wheel here at the fair. All he has to do is push her out from the top, and there's a net of course, but when he pushes her out, there's no net and she can't forgive him for her paralysis although he was taken advantage of by the Korean cameramen.
Why not make the 'antag' a deadbeat brother/sister that leaches off them, and guilt trips them into not kicking them out? Well after offering them a place to stay until they get back on their feet.
