"My returns my first 12 years were much better than my returns the last five years," he said, but his experience has deepened.
"I'm a massively better investor today than I was my first five and my first 10 years. And yet my returns don't reflect that. And so that's a question I'm always grappling with," he said.
The public is most familiar with Tilson from his appearances on the CBS news program "60 Minutes." He spoke on the program about the mortgage crisis in 2008, and more recently about his concerns over toxins in materials used by Lumber Liquidators.
He is also a prolific writer — of two books, a value-investing newsletter, many blog posts discussing stocks on investing websites and frequent group emails on topics ranging from his investments to his fitness escapades (he competes in Tough Mudder obstacle races, rides his bike everywhere and has plans this summer to climb the Matterhorn).
Some complain he uses the media attention to promote his investments. But that attention can work both ways.
"Because I'm a very public figure, everybody knows what my returns are," he said.
Tilson was up nearly 17 percent and 14 percent in 2013 and 2014 respectively, only to lose ground again last year, by about 7 percent. His fund is currently about $70 million in size.
"The last 12 to 14 months have been crummy," he said.
"Every bone in my body wants to quickly get back above my high-water mark," he said. "I'd feel better. My investors would be happier."
But he added he has to "consciously try and resist" the urge to swing for the fences using leverage or making hugely concentrated bets in order to get there, which would only make matters worse.
"I can survive. I'm 49 years old," he said, "and I plan to be doing this another 50 years. So I'm 18 years into a 68-year-run, is the way I view it. I'm playing the long game."
Indeed, his single-biggest position is in Warren Buffett's stolid Berkshire Hathaway, a company and a man he has written much about and studied closely. The short positions, like Lumber Liquidators or Herbalife, are a much smaller part of his portfolio, although they often attract the most attention.
These short positions are critical to his strategy, though. "It's sort of disaster insurance," said Tilson. "Shorting saved my butt in 2008. … Shorting kept me in the game. It generates cash when the market's crashing. And that's what you want when the market's crashing — cash."
That's not to say he'd recommend it to just anyone.
"My advice to most people is don't short stocks," he said. "It's a very, very difficult business. And you can really get clobbered," like he has shorting Tesla.
But it's still necessary for well-functioning markets, he said.
"If I bring to light a company that's poisoning customers, defrauding investors, or something like that, … there just aren't enough regulators in the world to keep up with all of the fraud and malfeasance that goes on out there, particularly in the little nooks and crannies of the market," said Tilson.
Indeed, some argue that short-sellers could have helped spot problems at troubled blood testing start-up Theranos, had it been public.
"I think more people should do it and should speak publicly about it," Tilson said.