I think James is too "raw" (as someone else said) to actually see "Isabella's true colors." In his letter he's angry with her, but I think he hasn't interiorized it yet. He writes "...happy for me had we never met! I can never expect to know such another woman!" He still sees her as perfect; he loved her (an immature, idealized love, of course) and he's working at seeing her "true colors" but hasn't reached that point yet.

Isabella, a bit more worldly than poor James, was taken in by one considerably more worldly than she. I think Capt. Tilney was the real villain of this trio. I do think that she wants James back because she threw away a good thing, and recognizes it now. She writes to Catherine, "...he is the only man I ever did or could love, and I trust you will convince him of it." A sad little statement, but James deserves better.