Complex sentence: Lennie, puzzled and scared, decides to leave him alone and to find a cave on his own, but George stops him by saying that Aunt Clara would not like Lennie walking by himself, and indeed wants him to stay with George.
Split into simpler sentences: Lennie, puzzled and scared, decides to leave him alone and to find a cave on his own. Just as Lennie is about to walk away, George stops him by saying that Aunt Clara would not like Lennie walking by himself, and indeed wants him to stay with George.