Social rules may have altered but human emotions don't change. I think there are few individuals who's reaction to being dumped by someone they were really in love with would be a philosophical "Oh well, if they don't want me then that's OK".

Some react to being dumped like that with an orgy of self doubt. Others defend themselves against the hurt with anger against the person who had hurt them, and this is what Wentworth does. "She is weak, a feeble character. She treated me wrong," etc. It is not surprising that this is how he responds, given how his pride and self-belief are such necessary and essential elements of his character.