What is Lennie afraid of?

In the first chapter of the novel Of Mice and Men, George blows up at Lennie ostensibly over his companion's offhand observation about the beans:
"I like 'em with ketchup."
The fact that George could react so explosively to such a simple statement takes Lennie completely by surprise. George begins with the following:
"Well, we ain't got any," George exploded. "Whatever we ain't got, that's what you want. God a'mighty, if I was alone I could live so easy...."
During this long tirade, and without a paragraph break, Steinbeck inserts a description of Lennie's reaction. Lennie seems bombarded by the onslaught of bitter words. There are too many words and implications for him to process.
Lennie knelt and looked over the fire at the angry George. And Lennie's face was drawn with terror.
This terror will have tragic results. Lennie at least understands from George's angry tone and the look on George's face that his friend and guardian is getting fed up with their relationship. In the next-to-last chapter, Lennie is motivated by the same terror when he accidentally kills Curley's wife in the barn. He is sensing that this incident is the last straw. George will go ahead and do what he said he would like to do in Chapter One: he will get rid of Lennie for good. George might simply pack up and leave--and Lennie would never know how to find him.
"Oh! Please don't do none of that," he begged. "George gonna say I done a bad thing. He ain't gonna let me tend no rabbits."
Lennie had suggested to George that he might live in a cave up in the mountains. But he knows he would be lost without George. The whole big world would seem like a mystery. Lennie wouldn't know how to survive for a day. No one would hire him.
Earlier, Crooks had sadistically brought out Lennie's terror by suggesting that George might not come back.
Crooks face lightened with pleasure in his torture. "Nobody can't tell what a guy'll do," he observed calmly. "Le's say he wants to come back and can't. S'pose he gets killed or hurt so he can't come back."
Crooks nearly gets himself killed by his suppositions. He has hit on Lennie's great terror--that of being alone in the world and forced to fend for himself.