The `Shame` singer claims he has only just discovered that what he thought was depression was a different condition and says his life has dramatically changed since he was diagnosed.
He said: "I thought I was lethargic and depressed as a person. I thought that was my make-up.
"I found out a few months ago that I`d been ill, completely by mistake, and I don`t want to go into the whys and wherefores but what I had made you lethargic and made you depressed and I`d had it for years and not known and I`m on treatment for it. It`s changed my life."
The 36-year-old pop star also spoke of his struggle to deal with fame, admitting he went "mental", but thinks it was to be expected because of the level of success he achieved as a member of Take That and later as a solo artist.
He told British radio station Magic 105.4: "I don`t think anybody`s responded to it particularly well when they get to a certain level.
"Elton John went mental, Elvis (Presley) went mental, I went mental. Everybody at some point, at some stage, goes bonkers."
Speaking about his debut solo single, a cover of George Michael`s hit `Freedom` released in 1996, Robbie admitted he should have taken time out of the music business to address his alcohol problem when he quit Take That in 1995 to go it alone.
He said: "I recorded `Freedom` to get the record company off my back.
"I was in a bad, bad way. It was pre-rehab and I`d just finished off the top drawer in the mini bar - the whole top drawer, all the liquors and spirits - then started the video.
"I was in a very, very bad way so I wasn`t making the best choices as you can imagine. I realised that I`d got to go to rehab, I was an alcoholic."