The Strictly Come Dancing star was speaking to Piers Morgan on Life Stories, in an episode that will be screened tonight (May 4) at 9pm on ITV1.
Donovan described how he ended up in hospital after a drug-induced seizure during Kate Moss's 21st birthday party in 1995 at LA's The Viper Room, saying: "I'd been in Sydney taking drugs and then I flew to LA. I got hold of some coke there, I'm jet-lagged, it's an extremely strong consistency of the drug and I spent my entire time in the hotel room. The only time I went out was to the party.
"It was an embarrassing situation to be in but I was a drug addict at the time."
The 'Especially For You' singer said he continued using cocaine immediately after leaving hospital: "I just went back to the hotel after they'd checked me out of the hospital and got back on it.
"You don't think rationally when you're on drugs. They control your world. It's sad, extremely sad."
Donovan added that he still has to fight to stay clean, commenting: "Temptation is always there with everything in life."
The star recently announced that he is "delighted" to be returning to the 'War of the Worlds' musical in a new role as Parson Nathaniel, adding: "It's going to be a whole new show and a whole new experience."