"Afterwards, obviously there's the physical scars that I've got. But then the mental side of it," he told Jonathan Ross.
"I didn't leave the house for probably six or eight months. The only times I was leaving the house was for medical appointments and surgeries," he said.
The former England all-rounder also revealed he had to have "about five or six goes at leaving the room" and "have a chat with myself in the mirror" to help overcome his anxiety before returning to the world of cricket as a coach.
"I'd not shown myself without a face mask to anyone. It was like starting again."
In a trailer released on Thursday for the 90-minute Disney+ documentary, Flintoff describes his "vivid" memories of the crash and says he can remember "everything about it".
A still image from shortly after the incident is shown, depicting staff and crew attending a crashed vehicle at Top Gear's test track at Dunsfold Park Aerodrome in Surrey.
Flintoff said filming the upcoming documentary was "quite strange" at first.
"Since the accident, had the flashbacks, the nightmares and things… you're talking about it, you're talking about it quite a lot," he said.
"I enjoy watching the cricket bits, wish there was more of them in it. The hardest part is seeing people talk about you."
He added: "I retired so long ago, it almost seems like a different life. Like watching from the outside."
Flintoff's family, Gavin & Stacey creator James Corden, and comedian Jack Whitehall also appear in the trailer for the documentary, which is streaming from 25 April.