This is a father whose son is suffering, and he is desperate. Jesus tells him everything is possible for the one who believes, and the man blurts out the most honest prayer in the Bible.
I do believe. Help me overcome my unbelief. He holds both at the same time, faith and doubt in one breath. He does not clean it up before he says it.
And here is the part that should change how you see your own doubt: Jesus healed the boy anyway. He did not wait for the father to reach perfect certainty. He met the faith that was mixed with doubt and honest about it.
So you do not need to fake a confidence you do not have. Bring Jesus the real thing: I believe, help my unbelief. That is not a failed prayer. It is one He answers.