If the person's expectations were strongly influenced by other people (such as really working up how great something is, so much that the person believes it must be great regardless of whether or not it is), there are two other possibilities:
in a state of refusing to believe something that is true. Mary was in
denial about her illness and refused treatment. Tom doesn't think he's
an alcoholic because he's still in denial.
The person simply refuses to believe something isn't true, and this is usually attributed to the idea that it's a defense mechanism because accepting they were wrong is more threatening than just distorting their perception.
brainwashed (as a result of brainwashing)
any method of controlled systematic indoctrination, especially one
based on repetition or confusion: brainwashing by TV commercials.
In this case the person was so strongly influenced into believing something is true that they will seemingly reject any perception, argument, or experience that indicates that things are not as they believed.