What does frodo see when he and sam reach the road on Mount Doom, and what is the effec on him?

Sam and Frodo both crawl up slope to Sauron’s Road. Frodo turns east and see the Eye. This causes him to fall. Sam carries him again.

Mount Doom is the end of the journey, and that is what Frodo sees. He sees the end of a quest; the necessity and difficulty with which he'll have to throw the ring into the pit, and he sees the possibility of his own end.