Have you been manipulated, lied to, controlled by a toxic person? The person may be a psychopath (antisocial), a narcissist or a borderline. They've betrayed, exploited and misled you. They've seduced you with their charm and beguiled you with their lies. They've stolen your money, your life, your heart. And you're left wondering, "How could I have been so stupid?"
I am reminded of an old Spanish saying that I absolutely love,
"The lion believes that all are like him."
An honest person expects people to treat them honestly. Someone who is not a thief may not lock up their belongings, because it does not occur to them that other people would steal from her.
Naturally, you need to check yourself for a susceptibility to flowery speech and manipulation. If you have a low self esteem you may be especially susceptible to someone showering you with compliments while they simultaneously mistreat, dismiss or disregard you.
But aside from that, one of the struggles of people who are dealing with psychopaths, narcissists or other manipulative people is the nagging question, "How could I not have seen this?", "How could I have been so stupid as to pick out this person?", "Why am I so gullible?" If you are really kicking yourself thinking you were inordinately naive or dumb for having been duped so completely you might find the following explanation helpful.
I found this description of psychopathic manipulation in an article on manipulation and controlling behaviors:
Psychopaths know well and have nothing but disdain for the characteristics of good-natured people and use those very qualities — including most people’s willingness to trust and afford others the benefit of the doubt and the conscientiousness most people have and discomfort they typically have when they think they might be the cause of anyone else’s pain — against them. Possessing a narcissism so malignant (“Narcissism: Pathological Self-Love”) that they consider truly decent folks as inherently weak and inferior, they feel “entitled” to prey on such folks, and deliberately play on their sensitivities and sensibilities to con, exploit, and otherwise victimize them. Worst of all, they do these things for the pure pleasure of it. It’s not fear, insecurity or emotional pain that drives them, just an incapacity to care and a craving to dominate.
Your gullibility and naivete is normal - because you are an honest person. One other thought you might want to consider. The alternative to being gullible and naive may be worse - to become cynical, jaded and suspicious of everyone.