It was 2am and my mind was still replaying a conversation I'd had with a colleague that afternoon - what I'd said, what he'd said, what I should have said. My heart was racing, the clock was ticking and, as the birds started singing, I realised I'd wasted any chance of sleep.To put this into context, I was 40, (generally) happily married with two healthy children, a solid group of friends and a reasonably successful career as a writer. Yet my mind was all over the place. I didn't just lie awake, worrying about things I couldn't change. I made diet and exercise plans, then abandoned them. I felt anxious, on edge, unable to relax.I wasn't suffering from anxiety or depression as such - certainly nothing serious enough to see a psychologist about - but I was worried about what this constant state of low-level stress was doing to my body and mind, which increasingly seemed to falter at the slightest of forces.Just this week, yet another study (at Pennsylvania State University) linked stress with obesity, heart disease and cancer. The kicker? It's not stress itself that causes these problems, it's how you react to it. Which, if the answer is "not well", just gives you more to worry about.The big new buzzword in self-improvement circles is resilience - the mental muscle that makes you emotionally tough enough to bounce back and carry on. So, how do you develop this emotional grit?Trawling through journal papers, meeting academics everywhere from Newcastle to New York City, and talking to everyone from a US Marine to a world champion kickboxer, I discovered that resilience isn't just something you are (or aren't) born with, it is something you can strengthen. Here is what I learnt:First, get stressed.Dennis Charney, Dean of the Icahn School of Medicine at Mount Sinai, New York, has spent decades studying resilience - interviewing those who have survived everything from natural disasters to being tortured as a prisoner of war - and is a big believer that "stress innoculation" builds psychological strength. Pushing yourself out of your comfort zone (but not too far) will develop both the confidence and the toolbox of tricks you need to tackle life's slings and arrows.Take extreme sport enthusiasts - I'd always assumed they didn't feel fear. But a team of British psychologists who sat down with 15 of them, both men and women, discovered the opposite. When asked what was going through his mind when he leaped from a cliff, one base-jumper replied: 'Please, God, don't let me die.'All the participants said they felt fear, but controlling it left them more resilient.Second, be positive.Easier said than done, but when I met Ian Maynard, a sports psychologist who's worked with Team Great Britain at various Olympic Games, and is now director of the Centre for Sports and Exercise Science at Sheffield Hallam University, he told me that mentally tough people are invariably "glass-half-full types".Martin Seligman, director of the University of Pennsylvania's Positive Psychology Centre, has a tried and tested technique to increase optimism levels: write down at the end of each day What Went Well, and why.The idea is that this helps you to focus on the good things about the day, rather than the bad, and to appreciate your role in helping to bring them about.Third, foster friendships.People who are lonely are more likely to be depressed and have a 30% higher risk of premature death. What matters is not how many friends you have, but how socially connected you feel. And thinking more kindly about people - concentrating on their strengths, rather than their flaws - is a more effective way to build meaningful social connections than simply meeting more people.Fourth, hit the "mind gym".Like most people, I knew exercise stimulates the release of endorphins, which make you feel good. The Yale Stress Centre's resident exercise specialist, Matt Stults-Kolehmainen, found that people who exercise regularly enjoy enduring benefits - calming down faster after a challenging event than non-exercisers, with a quicker and steeper drop in levels of the stress hormone cortisol.At Columbia University, Carol Ewing Garber has determined how much exercise you need to do for maximum psychological benefits: 150 minutes of "moderate to vigorous" exercise a week, which Stults-Kolehmainen suggests breaking into five 30-minute chunks.Fifth, accept that stress can be good for you.I feel better now that I exercise regularly, do things I was scared of, think positively and sleep better. I have come to enjoy the buzz of even the slightest success. That is not to say I don't still get stressed by setbacks at work or family squabbles, but now I am emotionally tougher, I recover more quickly...