Someone believes something is going to be a certain way so they think it is? [closed]

I believe the accepted term is confirmation bias:

Confirmation bias, also called confirmatory bias or myside bias, is the tendency to search for, interpret, favor, and recall information in a way that confirms one's preexisting beliefs or hypotheses, while giving disproportionately less consideration to alternative possibilities.

I think you're looking for self-fulfilling prophecy:

a prediction that directly or indirectly causes itself to become true, by the very terms of the prophecy itself, due to positive feedback between belief and behavior.

[Source: Wikipedia]

I like delusional, but it's somewhat context dependent.

[Webster's:] Delusional - A false idea; a belief in something that isn't true.