Jonas might be feeling dread or consternation ("fear resulting from the awareness of danger") about his upcoming training as a Receiver, especially since the Chief Elder had warned him about the indescribable pain. But these feelings are not the same as the discouraging sense of disappointment at discovering that his new schedule would leave him no time to play.
But he was a little dismayed that his schedule left no time, apparently, for recreation.
having many complexly arranged elements; elaborate
The intricate designs on the cloth represent the complex memories that Jonas will receive; the designs were embroidered into the cloth with a needle, which parallels the painstaking transfers of the memories through the Receiver's hands into Jonas's body and mind. Just as the cloth contains the intricacies for the functional bed underneath, the Receiver holds the complex memories so that the rest of the community can work and sleep more easily.
The bed, in an alcove at the far end of the room, was draped with a splendid cloth embroidered over its entire surface with intricate designs.
The phrase "I failed" turns the word "successor" into a pun ("success" and "successor" come from the same Latin words "sub" which means "after" and "cedere" which means "to go"). The Giver was unsuccessful in training his successor, who's supposed to come after him and replace him as the Receiver. Even though one can inherit the trait of light eyes that enables perception and reception, s/he must be chosen and trained for the title and office of Receiver.
But that does not mean I am perfect, and when I tried before to train a successor, I failed.
Almost everyone in the community has dark eyes, which represent their inability to see beyond the present. But Jonas, Gabe, and the Giver all have different, lighter eyes, which enable them to perceive and receive the world in all its forms and colors throughout history.
You should be able to perceive the name without being told.
“The Chief Elder told me—she told everyone—and you told me, too, that it would be painful. So I was a little scared. But it didn’t hurt at all. I really enjoyed it.” He looked quizzically at the old man.
"Assimilate" also means "make alike" or "become like one's environment" (Latin "similis" means "like"), which would be a desirable action in a community that "went to Sameness." The Giver is not exactly sure what happened to the memories when they were released, and he doesn't explain how they got assimilated. They couldn't go back to him, so they could have somehow just slowly blended into space and into the people's minds so that they were no longer shockingly different images and emotions.
“They really suffered for a while. Finally it subsided as the memories were assimilated. But it certainly made them aware of how they need a Receiver to contain all that pain. And knowledge."
He watched the landscape for glimpses of the green that he knew was embedded in the shrubbery; when it came flickering into his consciousness, he focused upon it, keeping it there, darkening it, holding it in his vision as long as possible until his head hurt and he let it fade away.