why did the guy say "i do believe" but then in the next breath said "help me overcome my unbelief"? which one is it?
Both.
Jesus said
Mark 9:23
23 Jesus said to him, “If you can believe, all things are possible to him who believes.”
So Jesus is saying "If"
The man replies I do believe but there seems to be can overcome the "If"
So there is a nagging doubt.
Let's not forget that the boy was possessed from childhood.
So in a sense the father is saying I believe but part of me is struggling with it. Please help me. You say if and I do if, help me change from if to it's true.
It's not dissimilar to what we experience in our walk.
Simple example.
Jesus said that God loves us as much as he loves Jesus.
That is true but how many Christians believe that?
My experience is not many.
So the question asked is "Jesus if that's true then please help me to accept this truth"
It's accepting the truth. Replacing the "If" of it might true with the reality that it is.
That is why the father said "I need help to overcome the unbelief that I'm struggling with"