Gentle reminder that yes, Riku was able to conquer his darkness in contrast to Terra, but while Riku has been forced to use his darkness to fight his friends, Terra was forced to use it to save his friends.
Riku grew into his darkness and embraced it willingly, in that way he had way more control over it. Terra on the other hand struggled with it, having been told all his life that darkness is bad and then suddenly it’s the only thing he can use to save Ven, and later on, Aqua. This internal struggle is exactly what Xehanort used to get into Terra. So by no means does that mean that Riku is better or stronger, their situations are simply fundamentally different.