No.
In my humble experience, affirmations start working anyway. And it seems that's the answer you have experienced to.
(If one goes into the details, it is possible it works because some subconcious part of us believes in the affirmated statement, underneath fear and self-doubt etc, but for all practical purposes; no you don't have to believe, just state it as a fact, and the universe responds).
But it's faster if you believe. And cooler things can be manifested with solid intent
Hogwarts-magic-style.
What happened to me, was that it worked when I just allowed for the possibility that it
could work. Then I started believing that affirmations work, and therefore that as soon as I said something out lout they were beginning to manifest: They might not rationally be true
at the moment I spoke the words, but they were true in some other 'sphere' that just needed some time to connect to mine, and therefore it was actually true.
(That was
not easy to describe...!) But you got my short answer up top... :)