what is the ending of the novel?

I think there is some closure for Jim here. It had been twenty years since Jim promised to come back, twenty years of his not yet fully accounted for. When Jim returns he finds what he decides should be the natural course of life yet still singularly Antonia,

"She was a battered woman now, not a lovely girl," he says of Ántonia, "but she still had that something which fires the imagination, could still stop one's breath for a moment..."

Jim finds beauty amongst the harsh debris of his past. He comes to terms with what he needed to. He decides his past is beautiful in its imperfectness and its splendour; he sees all of these things in Antonia's eyes.