The 24-year-old France midfielder says he asked the Gunners for a new long-term deal at the end of the 2006/07 season, but was told instead to prove his worth to the team.
He went on to be a major influence on Arsene Wenger's side, but agreed to join Milan on a four-year contract earlier this month.
"If Arsenal had offered me a contract last summer, I would still be their player now," Flamini told The Sun.
"Last July I told Arsenal I wanted to extend my contract so, of course, I would have signed if they had offered me a deal - but they didn't.
"They said they wanted me to prove myself first.
"It was difficult for me at the beginning. I had to see it as a challenge but I won't lie, it was not easy. You have to be mentally very strong to cope."
Flamini admits it was a big decision to leave Arsenal, but says he opted to join Milan because of the size and stature of the Italian club, despite Arsene Wenger's assertion that he left for more money.
"I've enormous respect for Arsene Wenger, but I did not leave for the money," he said.
"I'm leaving for Milan because it's one of the biggest clubs in the world - with millions of fans everywhere, an enormous history and one of the biggest trophy cabinets in the world."