The Brazilian international was signed by manager Arsene Wenger to provide competition to left-back Kieran Gibbs, but he struggled to find any consistency and was loaned out to Gremio earlier this year.
"It was [in England] that I learnt I was a full-back, whose first priority is to defend properly and to get forward only when it's necessary," he said.
"I made mistakes, which were more to do with me getting distracted than anything else. You have to maintain maximum focus because there are thousands of players who want your place."
Reports have claimed that the Gunners will allow the 30-year-old to leave the club on a permanent basis this summer.