The opera singer, who came second on the show with partner Charlotte Jaconelli earlier this year, described his success on the talent contest as a "miracle".
> Britain's Got Talent Jonathan & Charlotte stream new album online
"I felt that life was pointless. I was suicidal. I felt like I was on a roller coaster that only went up and I was just waiting for a drop. A catastrophic drop. It did come eventually and it was not pleasant," he told The Sun.
"I'd get home, collapse on my bed and then wake up at one or two in the morning. I'd just be sitting in my room thinking, 'What do I do now?' My life just felt pointless. The whole thing. I felt like I was going to be at school forever and just die there.
"I had suicidal thoughts. It was a serious thing. I was a danger to myself. I would self-harm. I'd cut my arms and my torso. I thought about it constantly for two months. There were times when I could have ended it."
Antoine struggled to juggle his schoolwork with choir practice after he moved to a private school in Walthamstow, East London, on a music scholarship. He was also bullied and eventually suffered a breakdown.
"It wasn't right for me," he explained. "I couldn't find any time for myself between the homework, school and sleeping. I'd start at seven to do choir at this claustrophobic church. Then more rehearsals after school. There was no time for anything and I just couldn't take it at all.
"[Britain's Got Talent] has definitely helped me. I'm not a man of faith but this feels like a miracle. Now, I don't wake up at one in the morning. I come home at one in the morning. And I am tired now because I'm doing something I love to do. I feel very happy."
Jonathan & Charlotte release their debut album Together on September 24. Listen to their version of 'Unintended' below: