He said: "When you are linked with other clubs, knowing it's probably true, all you think about in that moment is wow, maybe you can play in the Champions League.
"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."