We Fools are firm believers that successful investing requires patience, but we also recognize that waiting too long can sometimes be a costly decision. I learned this lesson the hard way in 2016, as my patience caused me to miss out on buying a few companies that I believed could have been great investments, but for one reason or another, I never bought the shares. Predictably, shares of both companies zoomed higher during the year, causing me to miss out on huge gains.

Here's a closer look at two companies -- Duluth Holdings (NASDAQ:DLTH), and Exelixis (NASDAQ:EXEL)-- that I wish I'd bought in 2016.

A winning niche 

I was introduced to the niche apparel maker Duluth Holdings as an investment idea in March. This fast-growing apparel retailer designs functional clothing that targets tradespeople in rural areas. The company couples its unique product design with a sense of humor that has helped it to create a loyal following.

Image source: Duluth Trading Co. 

The combination has worked like a charm. Revenue and profit grew by 29% and 47%, respectively, from 2009 to 2015.  Better still, the company operates only 12 retail stores and two outlets nationwide, which gives it a tremendous amount of room for future expansion. 

All of that sounds like a winning investing idea, but since I tend to be wary of newly public companies, I decided to be patient and watch the story play out from the sidelines. Predictably, the company's growth rates have continued unabated, thanks to new store openings and a successful nationwide marketing campaign.

The markets have certainly noticed the company's success, as shares nearly doubled in 2016. However, my reluctance to buy shares of newly public company caused me to miss out of the gains.

A biotech turnaround

I've been following Exelixis on and off for a few years now. The cancer-focused biotech came public more than a decade ago, but it has largely been a disappointing investment. I tend to shy away from stocks that have a spotty history, so I failed to act when my fellow Fool George Budwell suggested that investors should buy the company in late 2015. At the time, he was excited about the potential approval of the company's drug Cotellic as a hopeful treatment for advanced metastatic melanoma.

A few months later, Sean Williams expounded on his bullishness for the company's prospects. Shares had been on a stellar run, but he said the stock could continue to roar higher thanks to the expected approval of another drug, Cabometyx, for advanced kidney cancer. He also believed that the company was only months away from finding a partner.

Image source: Getty Images. 

You can guess what happened next. Exelixis went on to win FDA approval for both Cotellic and Cabometyx. Exelixis also found a partner. All of that caused the company's sales to grow rapidly.  

The never-ending string of good news caused shares of Exelixis to triple in 2016, making it the best of the year's top-performing biotech stocks. My inability to let go of the company's long-term history caused me to miss out on big gains.

Lessons learned

While my aversion to companies that are newly public or have a spotty history has generally served me well, I must admit that it can sometimes cause me to be overly cautious. But there are no called strikes in investing, so there's nothing preventing me from buying shares of either of these companies in 2017. I might be willing to do just that if the market offers us a discounted price on either of them.