The 26-year-old flight attendant, identified by the tabloid Bild as Mary W., was quoted as saying Andreas Lubitz, 27, had "burnout-syndrome" and she had been worried about his increasingly erratic behaviour.
The woman said Lubitz would have horrible nightmares and would wake at night screaming, "We're going down."
When she heard about the crash in the French Alps — which killed all 150 people on board — she remembered that Lubitz had told her last year: “One day I will do something that will change the whole system, and then all will know my name and remember it."
"I never knew what he meant, but now it makes sense," she told Bild. She said they dated for five months last year but she decided they should end the relationship after "it became increasingly clear he had a problem."
The woman spoke of his "health problems," described him as being agitated when he spoke of his work, and remembered him saying he didn't think he could realize his dreams of becoming a long-haul pilot and captain.
She added, "He knew how to hide what was really going on and how to hide it from other people."
Investigators say they have found evidence that Lubitz hid an illness of some kind from the airline, including a torn-up doctor's note that would have kept him off work on the day of the crash.
The Wall Street Journal, quoting a "person familiar with the investigation," said Lubitz had been excused from work by his neuropsychologist for a period that included last Tuesday.
German prosecutor Christoph Gumpa said investigators, after searching the co-pilot's apartment in Dusseldorf and his parents home in Montabaur, did not find a suicide note or anything to indicate a religious or political motive for crashing the plane.
The Duesseldorf University Hospital on Friday said Lubitz had been a patient there over the past two months and last went in for a "diagnostic evaluation" on March 10. It declined to provide details, citing medical confidentiality, and denied reports it had treated Lubitz for depression.
Investigators believe Lubitz locked his captain out of the cockpit during a flight from Barcelona to Dusseldorf, ignoring the Airbus A320 pilot's frantic efforts to re-enter while the plane continued to descend for eight minutes at full speed.
Germanwings said both pilots on the plane had medical clearance, and it had received no sick note for the day of the crash. Medical checkups are done by certified doctors and take place once a year.
Parent company Lufthansa is offering immediate financial assistance of up to 50,000 euros ($68,600 Cdn) per passenger to relatives of the crash victims, Reuters reported on Friday.