Former U.S. President Jimmy Carter says he thought he had only weeks to live after being diagnosed with brain cancer a year ago. Carter and his wife, Rosalynn, spoke to a news conference Monday at a Habitat for Humanity construction project in Memphis, Tennessee. The 91-year-old Carter, a worldwide ambassador for the charity, is being joined by about 1,500 volunteers during a weeklong effort to build 19 homes in a low-income neighborhood near the city's downtown. Carter said he feels "pretty certain about my cure," but that the doctors are "still keeping an eye on me." The former president revealed in August 2015 that he had been diagnosed with melanoma that had spread to his brain. He underwent treatment with a new drug, Keytruda, and by last December, he dramatically announced that the cancer had disappeared. Four months later, he said scans showed he was free of cancer and was able to end the treatments. But during the peak of his fight with the illness, he had his doubts. "A year ago, I didn't think I was going to live but two or three weeks, because they had already removed part of my liver because I had cancer there,'' Carter said. "After that, when they did an MRI, they found four cancer places in my brain, so I thought I just had a few weeks to live.'' By last November, Carter was responding to treatment and was able to travel to Memphis for another Habitat for Humanity project. "By the time I got to here, I was putting on a kind of a false optimistic face,'' he said. The former president wore blue jeans, a hard hat, a tool belt and a red bandanna around his neck as he hammered nails into the wooden frame of a house he was helping build. The project involves constructing one- and two-story houses. Moving amid the skeleton framing of the house, Carter appeared energetic and sure-footed as he talked with workers and used a level to make sure wooden beams were properly installed. Jonathan Reckford, CEO of the Atlanta-based charity, said Carter and his wife have donated their time to help build and improve 3,944 homes in 14 countries. The onetime peanut farmer said he plans to continue building houses next year, in Canada.