Griffith realises that in his state he will never accomplish anything, his dream all of his past and future are slipping, all he fought for, all the people he sacrificed for the sake of his dream... He's finally realising the weight of it all, how he promised everyone that in exchange for their lives he would accomplish his dream, and then everything would have been worth, all the lives that offered themselves to him would have been honored.
But he can't. He can't have Casca, he can't have his dream, he can't even wield a sword, he won't even die properly.
All that weight has finally come crushing him all at the same time, the consequences for his action.
He MUST achieve his dream now,
AT.
ANY.
COST.