You have a significant retirement portfolio. You're an experienced investor. You've done pretty well at picking stocks. You probably even own a few of Zacks Top Retirement stock picks like:Vistra Energy Corp. (VST), Camden National (CAC) and First Financial Corp. (THFF).If this sounds like you, then here's a question: With your background and skills, should you manage your own retirement investments?Perhaps...if you're the "one in a million" investor who can expertly manage risk and maintain unflinching emotional control in volatile markets. But for most, there may be better strategies to achieve long-term retirement investing goals.That's because the risk - reward scenario and investing approach is completely different for long-term wealth building and active stock trading.Managing Retirement Investments: Stock Picking vs. DiversificationWhile stock picking can potentially generate outsized returns, its excessive concentrated risk can present huge perils for retirement investors.A study done by Hendrik Bessembinder of equity markets spanning nine decades revealed that only 4% of the best-performing U.S.stocks produced all the market's increases. The rest were flat - the gains of the following 38% were offset by the losses of the bottom 58%.For even the most talented stock pickers, the odds for long-term success are slim.Is it Possible to Invest "Rationally"?Most people think they can make rational investment decisions, but research indicates the opposite is often true. Investors followed in a DALBAR study performed significantly worse than the S&P 500: For the 30 years between 1986 to 2015, the average investor earned just 3.66%, whereas the S&P 500 produced a 10.35% return.Importantly, this period included the 1987 crash and big bear markets in 2000 and 2008, but also the bull market of the 1990s.This study indicates that one key explanation behind investor underperformance is attempting to time volatile markets - and that irrational emotional biases are likely to compound investor botches.Interestingly, even savvy traders tend to underperform because they can't help but allow emotions to drive investment decisions. They may be overconfident and misjudge risk, latch onto a price target, or perceive a pattern that isn't there. This "behavior gap", over the long-term, can be catastrophic with potential underperformance of hundreds of thousands of dollars sabotaging your retirement.What It All Means for Retirement InvestorsYour retirement portfolio should be managed with a strategy of performance over decades - not days, weeks or quarters. Most self-directed investors tend to fall short when it comes to long-term results.Does that mean you should quit trading? Not really. One plan is to take 10% of your investable resources and trade to create alpha and look for outsized returns.However, the major part of your wealth - those assets reserved for retirement - ought to be invested utilizing a more careful, conservative, risk management strategy to produce steady, compounded returns so you can securely achieve your retirement objectives.Do You Know the Top 9 Retirement Investing Mistakes?Whether you're planning to retire early or not, don't let investing mistakes derail your plans.If you have $500,000 or more to invest and want to learn more, click the link to download our free report, 9 Retirement Mistakes that will Ruin Your Retirement.This report will help you steer clear of the most common mistakes, like trying to time the market, lack of diversification in your portfolio, and many more. Get Your FREE Guide Now Vistra Energy Corp. (VST): Free Stock Analysis Report First Financial Corporation Indiana (THFF): Free Stock Analysis Report Camden National Corporation (CAC): Free Stock Analysis Report To read this article on Zacks.com click here. Zacks Investment Research