The ex-Take That star checked into the Meadows clinic in Arizona two years ago.
'Robbie Williams does not go into rehab just if his life gets a bit out of control,' he says.
'It was damn serious. Death had to knock on my door to help me with my decision.'
Robbie, 35, reckons girlfriend Ayda Field, 29, has helped him turn his life around.
'I'm aware I have a self-destructive character,' he tells German magazine Stern. 'But does this hold me back from being happy? No...
'I am a very happy man at the moment. I'm now in love with someone who returns this love as strongly as I love her. I've never experienced that before.
'That's what makes me so careful in this relationship, because I don't want to hurt Ayda.'
Robbie's single Bodies is out on 12 October and his album Reality Killed The Video Star follows on 9 November.
Where to next?- Take That: The comeback kings
- Watch Robbie Williams take a tumble on stage
- Robbie Williams on rejoining Take That