Self-delusion occurs when a person tricks themselves into thinking something, which usually isn't true. This might happen when something very bad happens, like a close family member or friend got into an fatal accident, and you have no clue if they're okay or not, because you didn't know it was fatal. You may try to make yourself believe they are okay, when they are sadly not. You delude yourself.
Sally thinks her dog'll be okay. She really has deluded herself. (self-delusion. She deluded herself into thinking her dog would be okay)
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