"I think it surprised me that it would mess my head up so much. I was surprised that I could be brought down by it.
"I am 29 and not many players at my age get the chance to go to a massive club in the Premier League. That's what probably got me down when it didn't happen for me."
McCarthy, who has yet to commit to a new contract and could still leave next summer, was so out of sorts at the start of the season that boss Mark Hughes dropped him after a defeat by Portsmouth at home in September and it was the wake-up call he needed.
McCarthy said: "I told myself I had to put in the same performances as I did in the beginning at Rovers, so people would not say my first season was pure luck."