The Hardest Part for Ichigo
The hardest part for Ichigo is not knowing. It's being pulled into situations such as these that completely eat at his will to go on, tearing piece by piece his will to live; and at the same time, it's the lack of knowledge he has inhabited in comparison to his enemies. It's the lack of knowing he can't possibly catch up that fast.
The hardest part for Ichigo is a mind numbing knowledge that runs down his bones, that sears at his heart. He knows he has got to save Rukia, has got to get to her, because if he doesn't, no one will. He's determined to save her because he owes it to her. He won't let her die. He can't let her die.
The hardest part for Ichigo is not knowing what he knows deep inside. It's there somewhere inside of him, but as he fights, as he loses and subsequently gains retrieval of the upper hand… as he dredges up powerful memories that aid his fighting, he constantly is learning. He doesn't know what he's capable of, and if he wants to found out, he must be void of fear.
He knows this, that he's learning as he's fighting, that his enemies are merely using what they already know, that his powers will never catch up, that he might die.
He doesn't believe it, though, because he has a stronger will (than Kenpachi's love of fighting) on his side.