Barrett, capped 146 times by New Zealand, was overlooked for the first six Tests of the year before making his return off the bench against South Africa in Baltimore, where he scored a late try.
He finally earned a starting berth at fullback against Australia at Eden Park last weekend, helping New Zealand thrash the Wallabies 50-19 to retain the Bledisloe Cup, the annual trophy contested by the trans-Tasman nations, for a 24th straight year.
"It did challenge me mentally a little bit," the twice World Rugby Player of the Year said on Monday.
"Not being selected and just feeling a little bit off the pace. When you are in the 23 or the 15 you are in the thick of it, you feel that flow, the connections, the combinations and all of that.
"I would be lying if I said that it hasn't affected me, but the important thing is that I have just been able to bounce back and focus on what I do well and be consistent."
Barrett, previously first-choice flyhalf under former coach Scott Robertson, found himself down the pecking order after Rennie took charge, with 25-year-old Ruben Love preferred at number 10 and Damian McKenzie at fullback.
He was relegated to helping the match-day squad prepare at training, but said he had worked hard to regain his confidence.
Injuries to Love and Richie Mo'unga have since opened the door for Barrett, who hopes to retain his place for Saturday's second Bledisloe Cup Test in Sydney.
Barrett said Rennie had been forthright about his standing in the squad.
"He was telling me he was liking how I was training, he was happy with my contribution and it was clear that I wasn't up in the pecking order, so I had to really work for it or wait for my opportunity," Barrett added.
"We are both honest with one another, we know where we stand and there are no mixed messages."