I had to edit your question down to one question according to enotes regulations. We are told at the beginning of Chapter Sixteen very clearly that Jonas really did not want to go back to the Giver after receiving the memory of war and the accompanying pain that went with it. The narrator informs us that:
He didn't want the memories, didn't want the honour, didn't want the wisdom, didn't want the pain. He wanted his childhood again, his scraped knees and ball games.
And yet, in spite of what he wants, Jonas knows that he has been given a position of responsibility on behalf of the entire community - it is not just about him and his own wants and desires. This is why he does go back each day, because he recognises that "the choice was not his." In spite of how he is suffering and changing he knows that he has to go back for the good of the whole community - they are counting on him.