Ron is tested and forced to evolve in this book in a way that Hermione is not. His abandonment of Harry and Hermione in the forest is an act that we really don’t expect from him, and in order for us to forgive him and accept his return, we need to see, as with Harry, the visible proof of what his struggle really is.
Part of the reason for Ron’s departure (aside from the negative influence of the Horcrux, which only exacerbates problems that are already there) seems to be simple immaturity. Ron has always been well fed, both at home and at school, and is quite greedy about food. When they’re on their own, he expects Hermione to feed him and take care of him, showing that he’s still basically a child. The same dynamic applies to the quest. Ron is brave and loyal enough when he’s along for the ride on one of Harry’s adventures, but the idea that Harry doesn’t know what he’s doing makes him very uncomfortable, because Ron needs to be told what to do.
When Ron comes back, however, and we see his fears manifested by the Horcrux before Ron destroys it, we see that his problem runs deeper. Always playing second fiddle to Harry, Ron does not believe that he could actually be loved—not by his girlfriend, not even by his own mother. When he accepts that he is loved, he is able to grow up and take responsibility for his part in the quest, no longer needing others to prove their love by coddling him.