Why does George struggle with the description of the farm when Lennie asked him to recite it to him on the river bank?

Answer 1

The heartbroken George starts to struggle with the image of the farm at this point because deep inside, George had known that this dream is shattered without Lennie, it would never come true and so he can't picture it as vividly as he once could have.

Answer 2

George struggles with the description of the farm because it represents a dream that will never come true. He knows that with Lennie's limitations, they will never be able to achieve this idealistic vision of a quiet, peaceful life on a farm. Deep down, George is aware that this conversation is just a bittersweet moment of escapism from their harsh reality.