Western Union (NYSE:WU) shares soared 34%, year-to-date, trumping the S&P 500's gain of 18.5%. Despite the stock's success, however, it is time for me to bid it farewell.

On Jan. 9 2013, I made a CAPScall on Western Union. My investment thesis was simple: it was undervalued. At the time, the company traded at just 6.7 times forward earnings estimates. I believed that the company's scale, combined with low expectations, would reward investors over the long haul.

Though my pick more than doubled the market's returns during that time period, I'm not ready to pat myself on the back.

Straying out of my comfort zone
When I buy stock, I think of it like a business, and I plan to hold for the long haul. Likewise, I've been trying to practice this philosophy with my CAPS picks, too. Recently, however, I was scrubbing through my picks, and I realized I had to say goodbye Western Union. Low expectations or not, it just isn't looking like a business I want to hold for a lifetime.

Western Union's second-quarter results left me wanting. Revenue declined 3% year over year due to falling prices. Despite transaction growth, it looks to me like the company's pricing power may be waning and hampering the top line.

Meanwhile, expenses crept upwards, increasing 3% year over year. With revenue and expenses trending in different directions, the company's operating margin is suffering. Despite a gallant share repurchase program, reducing the share count by 9%, EPS and operating income were down 18% and 20%, respectively, year over year.

Reflecting on my initial reason for buying the stock, I think the purchase was a mistake. The focus of my investing strategy is on buying businesses that I feel comfortable holding a lifetime. Instead of analyzing that aspect of Western Union back in January, I was simply fixed on my belief that the stock was undervalued. With this pick, I strayed from my investing strategy. Though the outcome was great, I'm not proud of the underlying method I used to make my investment decision.

The ultra long-term approach
I subscribe to the belief that the best way to invest over a lifetime is to follow Warren Buffett's advice from his 1989 letter to shareholders: "Time is the friend of the wonderful business, the enemy of the mediocre."

Though Western Union still seems undervalued, it's not a business I can see myself holding in 10 years. For that reason, I'm out.

Fool contributor Daniel Sparks has no position in any stocks mentioned. The Motley Fool recommends Western Union. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.