Lohan, 27, said in the interview, which aired Sunday night on OWN, she is in "a different head space" and intends to remain "present and clear-headed and focused" after undergoing treatment.
"I feel whole again and I have such a desire to want to keep this feeling and stay this way, and I'm willing to do whatever it takes," the actress said.
She acknowledged an addiction to alcohol, which "in the past was a gateway to other things for me," but did not say what other substances she routinely abused.
So, what is she on these days?
"Vitamins," Lohan replied.
Asked whether she is taking any prescription drugs, Lohan answered, "No, no," but then she clarified, "I take Nexium because I have acid reflux."
Pressed about whether she thinks she can turn over a new leaf this time, Lohan told Winfrey, "Yeah."
Lohan was under court order to undergo in-patient treatment for substance abuse as part of her conviction for reckless driving and lying to police.
She checked into the Betty Ford Center May 3 but reportedly was unhappy with her treatment and requested a transfer to Cliffside Malibu. She wrapped up her treatment and left the facility last month.
People.com said she must continue out-patient therapy into late next year.
The "Mean Girls" and "Freaky Friday" star's once-bright career has been eclipsed by her battles with drug and alcohol addiction, motor vehicle offenses and other legal woes.
The Oprah Winfrey Network reportedly paid the troubled actress about $2 million to star in an eight-part docu-series about her efforts to remain sober and revive her career.