Pickup terminology. Oneitis is where a man, in his mind, exalts a woman to an absurdly high status, idealizing her as an angel capable of fulfilling all his emotional needs. This typically involves dwelling upon her in his thoughts several times a day and creating imaginary scenarios involving her tending to him in some way, or future situations where she is his girlfriend or wife, and going over past interactions with her to fuel this fantasy. It is pathological as it is not based in reality and is a type of maladaptive daydreaming. The fantasy woman usually serves as a kind of mother replacement.
This should be stopped immediately by setting intentions for positive win-win interactions with people emphasizing giving as well as receiving (see: Intentional Socializing) and by using daily mindfulness meditation to see through the maladaptive thought patterns and let them go in an unattached way.