] IMO Marianne has received a formidable blow. She is young, it´s her first love, and of course she believes in Wickhams feelings until she is proven wrong. She doesn´t want to be proven wrong, though. Therefore it takes a lot of time till she admits to herself that he has been deceitful.

I think there's a fair amount of evidence in the novels that JA herself felt that true love meant always wanting to think well of the beloved, even if things didn't work out. Even Elinor is glad that she can work things out in her own mind so that she doesn't have to think badly of Edward, though an outsider might be rather impatient with him (!), and in P&P, Jane asks Elizabeth to let her continue to see Bingley's behaviour in the best light possible, whereupon Elizabeth stops arguing with her.