Diana's primary significance is that she provides Dunstan Ramsay with a way back into society after his serious injury during the war. Through her kindness, humanity, and infinite patience, Diana humanizes the very shy and intellectual Dunstan, helping him to become a different person. Yet old habits die hard, and Dunstan cannot completely escape from his past, no matter how much help and support he receives from Diana. Dunstan's still in thrall to unpleasant memories of childhood, of his stern, hyper-critical mother forever chiding him for his many eccentricities. Although Diana's a much nicer person than Mrs. Ramsay, Dunstan is unable to forge a deep romantic connection with her, because her authoritative nature reminds him of his mother.
Dunstan may be incredibly grateful to Diana for what she's done for him, but he's not prepared to sacrifice his individuality completely. He needs to hang on to those unique characteristics of his that make him the person he is, that make him stand out from the crowd. It's one thing to smooth out Dunstan's eccentricities, but getting rid of them altogether is a different matter entirely, and so Dunstan cannot allow Diana to mold him, however gently, into someone he isn't.