The stock market has been booming, and we're now officially entering the third year of the current bull market. The S&P 500 (^GSPC 0.39%) is up by more than 64% since its lowest point in late 2022, and many investors have watched their portfolios explode in that time.

Even the strongest bull markets can't last forever, though. Roughly 28% of U.S. investors believe that the market will be headed south in the next six months, according to a November 2024 poll from the American Association of Individual Investors.

While that figure is lower than it has been in the past (more than 37% of investors felt bearish in August 2024), that's still roughly 73 million Americans who are expecting stock prices to drop in the relatively near future.

The good news is that the market is more stable than it might seem. However, there's one situation where you might need to worry.

It can be tempting to focus on how stocks will fare over the coming weeks or months, but the market's long-term performance is far more important than its short-term ups and downs.

Even in strong economic times, the market will experience fluctuations. It's also impossible for even the experts to predict precisely what stocks will do or when prices will begin falling. While this uncertainty can be alarming, the good news is that over decades, the market has a flawless track record of earning positive returns.

Now, this isn't to say that there won't be turbulence looming. There's always a chance that the market could take a turn for the worse, and some of those downturns may be severe. But historically, those who have held investments for at least a few years -- or, ideally, a decade or more -- have managed to come out the other side unscathed.

Say, for example, you invested in an S&P 500 index fund in January 2008. The market was already about a month into the Great Recession, but the worst was still yet to come. The S&P 500 wouldn't reach a new all-time high until 2013, and those years in between were rough. Yet by January 2018, you'd have seen total returns of more than 82% -- nearly doubling your money.

For a more extreme example, say that you had invested in the S&P 500 back in early 2000. The dot-com bubble burst was just beginning, and it would go on to become one of the longest bear markets in the S&P 500's history. Then as soon as the market recovered and began reaching new highs, it was hit by the Great Recession.

While it's not easy sticking it out through the difficult years, if you'd simply held your investment through all the ups and downs, you'd have earned total returns of close to 300% by today -- and that's despite the major market crash in 2020 and the more recent downturn throughout 2022.

The market will always face volatility, some of which may be severe. As difficult as it can be, that's the reality of investing in the stock market. But if you hold your investments for the long haul and stay focused on the future, you're far more likely to see positive total returns.

Staying invested for decades can go a long way toward protecting your portfolio, but you'll need to ensure you're investing in the right places. If your stocks are shaky, there's good reason to worry that they may not survive a market slump.

Even weaker companies may see their stock prices surge when the market is booming, and it's easy to get caught up in the hype and buy stocks that appear to be thriving based on their market performance. However, when the market inevitably takes a downward turn, these shaky stocks may not have what it takes to bounce back.

Now more than ever, it's crucial to do your homework and study up on every stock you own or are thinking about buying.

Double-check that they have strong business fundamentals, which can include everything from strong financials to a competitive advantage. Are there any industry trends that could indicate whether this company has room for growth? Has the company's management team shown signs that they can guide the business through rough economic times? If you find any major red flags, now could be a smart time to sell while prices are still higher.

Researching every company you own or are interested in buying can be tedious work, but it will pay off during market slumps. By making an effort now to ensure you're only investing in high-quality stocks with solid fundamentals, you can rest easier knowing your portfolio is as protected as possible against future market volatility -- regardless of when that volatility may strike.