Since entering the facility, the 48-year-old actress - the youngest person ever to win an Academy Award - has been visited several times by her father Ryan O'Neal, who is desperate for his eldest child to get clean.
A source told RadarOnline.com: "Tatum recently went back to rehab last month, and she is at a facility in Orange County. Ryan has been visiting her regularly because he is so concerned about her. They are working on their complicated relationship.
"Tatum is just really truly struggling with who she is as a person. It's very sad to see her still suffering from her addiction to cocaine."
Tatum - who has had a complicated relationship with Ryan over the years and were, at one point, estranged - has been in out and out of rehab throughout her life as she struggled with her drug addiction.
She previously dated the late Michael Jackson and in 1986 she married tennis player John McEnroe but didn't invite anyone from her family to the ceremony.
The couple - who have three children - separated in December 1992 and were divorced in 1994.
Following the divorce, Tatum's long-time drug problems re-emerged and she developed an addiction to heroin.
As a result of her drug problems, John obtained custody of the children in 1998.