How to master investing's three hardest lessons

— -- Q: What's the most difficult aspect of investing, and what are ways to master it?

A: If you ask 20 investors what part of investing trips them up most, you'll likely get 40 answers.

Investing can be difficult, especially if you're trying to buy and sell individual stocks. The stock market seems to defy the wit and wisdom of even the most experienced investors every trading day.

After answering thousands of readers' questions about investing, I'd say the things that trip up most people fall into three categories:

1. Getting started. Some investors have trouble saving enough money so they can invest. And once they cobble together the savings, they're so bewildered they don't know where to start.

The solution: Just start saving. Most people can save, if they just open a bank account. If you're interested in figuring out how much you should be saving, here's a link that can help.

2. Tracking performance. I'm often struck by how many people invest and stick with a strategy, even though it's not working and could be costing them money. Some investors chase hot stocks and only remember the winners, forgetting the losers that wiped out all their gains. Other investors get their earnings chewed up by fees and charges. Still others are too conservative and accept returns that won't meet their long-term financial goals.

The solution: Learn how to measure your portfolio's return. It's not that hard if you practice. Here's a guide on how to do it.

3. Knowing when to sell. If you're buying and selling individual stocks, you have probably experienced how difficult selling can be. Even if you buy the right stock at the right time, it's tough to know when to either lock in your gain or cut your loss. If you sell a winning stock too early, you might miss giant gains later. But if you sell a loser too late, the loss could be devastating.