Smith was taking part in the 2013 San Diego Comic-Con Nerd HQ panel at this year's event when a mother of a young female fan asked him why he was leaving the BBC's long-running science-fiction television drama series. Smith replied : “It’s hard to leave. It’s an awful thing to contemplate, it really is. It’s been the most incredible experience of my life.”After a moment's further thought, the British actor added , “Things like this and Comic-Con, it’s changed my life ... It’s really depressing. You’ve gotta go when you’ve gotta go ... I don’t know ... I don’t know! I’ve made a mistake!”Smith announced last month that he'd decided to leave Doctor Who after three years as the Eleventh Doctor. The BBC is currently searching for his replacement. The actor chosen to play the Twelfth Doctor is expected to be revealed some time in the next few weeks. He or she will make their first appearance in the iconic role in this year's Christmas special.Meanwhile, Smith's penultimate episode as the Doctor will be broadcast on Saturday, November 23.