I want to say that though fast,Armaan's transformation was not overnight.Let me remind you of the episode in which Armaan overhears Jassi telling the gang of her wedding...Armaan literally lost his balance and bent against the wall...that was the first big shock to him..he had always maybe taken as granted that Jassi was always Gulmohar's (or maybe his?,he didn't know).He used to get uncomfortable whenever he used to face Purab...Then came the part when Jassi blamed him...that was too much for him.we could see that Armaan was thinking (prob for the 1st time in his life) during those episodes (scene with Raj).
I agree the transformation should have been slower,but it was definitely not overnight.