LONDON, England - Mark Webber, now 37, has admitted fading motivation prompted his decision to retire from Formula 1.Austin and Brazil will be the Australian veteran's last two GP's in a career lasting more than a decade with nine wins and 40 podium visits. He's switching to Le Mans sports cars to head Porsche's prototype foray but admits he was already starting to think about retirement back in 2012."Porsche wanted me for 2013," revealed Webber. "I said I wasn't ready but during that year (2012) I started to think about a change. I had already been at Red Bull for a very long time, so you do think about doing something else. That's human nature."'NOT 19 ANY MORE'Webber admits he courted a move to Ferrari but ultimately decided during Christmas 2012 to quit F1. Another factor, he said, was his fitness."A lot of people have trained with me," Webber told Speed Week, "but most are gone after less than two years. I never had to be motivated to keep fit but over the past year the urge went away... I had to ask myself why and it becomes clear - 'Mark, you're not 19 any more!'"He admitted that, increasingly, the desire to do things other than F1 crept in. "In my case, the reasons are very personal," Webber said. "I want to spend more time with my family, my partner, my friends. Suddenly there are things on the radar that weren't there before. I had often heard other sportsmen and women talk about the problem of their motivation going away but I always thought to myself, 'What?' The fact is that it does go away!"Also, when you're 36, you think differently to when you're 25, and that doesn't just go for drivers. As an athlete, you put so much into your career. I wouldn't say that we make sacrifices but if you start thinking that, then maybe it is time to do something else. You have to believe it's all worth it."I'm enjoying the summer in Australia and then suddenly you have to go to Jerez for winter testing and you think, 'Hmm...'. I also knew that I wouldn't be happy if I stopped racing completely. You have to find a balance, something that stimulates you, and I've found that with Porsche."