flatter (oneself)
To hold a vain, self-congratulatory, and exaggeratedly high opinion of oneself and/or one's achievements. Yeah, don't flatter yourself—she only asked you out to dinner because she wants to ask you about your brother. Gaston likes to flatter himself that he's the best man in town, but everyone knows he's just a pompous jerk.
flatter oneself
Be gratified vainly by one's own achievement; exaggerate one's good points. For example, He flattered himself that his presentation at the sales conference was a success, or She flattered herself that she was by far the best skater at the rink. This usage is often put negatively, as in Don't flatter yourself-we haven't won the contract yet. [Late 1500s]