I don’t know if it is a popular saying, but “That which you focus on becomes your reality” is certainly popular with me. It ties in with how our expectations can become self-fulfilling predictions. The French say, “Beware of your dreams…”, in English we say “Be careful what you wish for…”.
Both these point to the idea that we might unconsciously create our secretly held desires, but it works both ways.
If our expectations are that everything will go pear-shaped, then we shouldn’t be surprised or feel let down when it happens. “Follow your dreams” is great advice, but beware of what lurks deeper down. It’s great to have a positive outlook, but that’ll do no good if your underlying beliefs say the opposite.