Investors often consider their biggest mistakes the stocks or trades that lost them money. Yet the reality is, it's almost always the great stocks investors sold too soon that cost the most. On The Nov. 11 edition of "The Wrap" on Motley Fool Live, host Jason Hall describes how he made this mistake with one stock, selling a company that he loved -- even as he was selling it -- entirely because he thought the stock had gotten too expensive. As a result, he missed out on enormous gains as the company continued to execute, and more investors bought into what could prove a once-in-a-generation business. 

Transcript: 

Jason Hall: I'm going to share a little "happier, smarter, richer" from my own investing experience. A lot of times we crow about our investing successes. We talk about a lot on Motley Fool Live about stocks that we love, that we see as having tremendous prospects to grow wealth. Like we talked about it earlier on The Wrap here today about stocks that haven't done well for us or places that we've lost money. But I don't think we even do enough talking about our biggest investing mistakes.

I want to talk about mine. This has really become more and more apparent as time has gone by. That is selling Shopify stock at around $200 a share back in, I don't know. What was it? 2018, I think I sold. I think it was in early 2019 is when I sold for around $200 a share because I thought that the valuation had gotten a little bit ahead of itself.

I was convinced that the market was going to give me an opportunity to get back in at a better price. If you look at the charts, and there's some revisionist history. Obviously, it's hindsight bias playing a role here because we all know how well it's done. But the point is, my thesis, I sold somewhere right about here. I don't know if you can see that in 2018, you see what it's done since then. This is a stock that would easily be by far the largest stock in my portfolio if I just simply have held and I'd done anything.

I never had questions about Shopify as an investing thesis. I never questioned that. As I was hitting the sell button, I was thinking about how great a CEO, Tobias Lutke is, and how great the company is, and their mission, and how well they execute. But I was entirely, entirely anchoring on valuation. The lesson that I want to share is we don't know what's going to happen in the next, however long, six months, a year. We don't know what valuations are going to do. We don't know how investors are going to consider valuations in these high-flying tech stocks and what they choose to do.

But when we anchor too much on valuation, when we own excellence, it usually leads to making the mistake of selling, because you're walking away from a lot of good reasons to own a great business. Now, that's not always the case. I mean, sometimes you do want to walk away for valuation, but if it's part of a more holistic thesis. I don't know if you guys have anything to add to that, but to me it just really gets back to when you sell great businesses on price, you're almost always making a mistake.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.