It begins with Whis telling Goku, as he's battling Moro, to shatter the crystal in Moro's head to defeat him. Goku is surprised by this news and asks "You think I can pull it off?" Then Whis has to give him a little pep talk, saying things like "You have already acquired divine power" and "Have faith in yourself" which leads to a shocked Goku contemplating Whis' words.

Goku then continues to sound unsure. With his next statement his words trail off, "Okay ... if that's the only way ..." and its not until Beerus steps in to threaten Goku comically, that Goku seems to gain some of his bravado back. In the end Goku does fail this first volley even with Vegeta's assistance and it's not until everyone else gets involved such as Piccolo, Goten, Trunks, Jaco, the Earthlings, everyone at the lookout, and most importantly Uub and Daikaioshin that Goku is finally able to succeed with their help.
So maybe it's a case of him properly accessing that he alone didn't possess what he needed to defeat Moro, but for me it's so odd seeing him being this unsure of himself. It's to the point that it almost feels out of character, which is crazy to say because everyone has doubts but Goku rarely shows them.
Does anyone know if he's ever doubted himself this much before? I can't think of any other times. I know one example some of you might say, but in that one I believe he displayed a lot confidence in the situation.