passages in the letter is;

I know not in what manner, under what form of falsehood he{Mr Wickham} has imposed on you; but his success is not perhaps to be wondered at, ignorant as you previously were of everything concerning either. Detection could not be in your power, and suspicion certainly not in your inclination.

Darcy is obviously claming down ,and is really complimenting Elizabeth and her character.She is not a suspicious person.

Even when he was dissapointed in love, he was generous with praise with regard to Elizabeth's character.