I don't think that's it. With what was at stake, Riku gave it his all - to the point he used the darkness when he lost.
It's much simpler imo to just look back at KH1 - Riku lost to Sora. Mental issues had to do with it this time, no doubt, but the fact remains that even when Riku was confident in fighting Sora, he still lost.
He wouldn't need to hesitate that much in order for Sora's Nobody who seems to be a much better fighter at the time of his fight with Riku than Sora was when he fought Riku in order for Riku to lose.
Well, yeah, normally that's what I'd think, too, since it's the logical thing for Riku to do, and there's no reason Roxas shouldn't be a better fighter than Riku on his own.
The problem is, what we get to see of the fight really doesn't back that up. Riku, still in his own form, strikes Roxas down with Oblivion - then, instead of doing anything to make sure Roxas doesn't get back up, he sticks the Keyblade in the ground next to the unconscious Roxas, then stands over him until he wakes up and slashes at him with the Keyblade he'd been using. It's just such a
dumb move on Riku's part, if he's actually doing everything in his power to win the fight. He could have easily ended it right there - pin Roxas' hands while he's knocked down and weaponless so he can't use Oathkeeper and put Oblivion to his throat, and he's not going to be in a position to fight when he wakes up. (It wouldn't have been particularly elegant, but, then again, the goal is to knock Roxas unconscious and kidnap him, so...)
Not to mention the fact that he doesn't even try to summon Soul Eater to help block Roxas' Keyblades, let alone try to continue the fight that he seemed to have been winning. Or the fact that, even after dropping him for a second time as Ansem, Riku just stands over Roxas again instead of doing something to bring him in.
Which isn't to say that Roxas couldn't have won on his own merits, just that Riku was sufficiently uncomfortable with his goal that he could have failed to take any opportunities to drag Roxas in during the first fight that might have existed.
