I’m in the process of consolidating all of my investment accounts at Fidelity. This isn’t because I think Fidelity is “the best”, but because I think they’re good and they’re certainly convenient. There’s a Fidelity “investor center” not far from my home. (In other words: I’m not endorsing Fidelity; I’m merely following my own advice to pick a good option instead of spending forever looking for the best.)

As I gather my various accounts under one roof, I’m also trying to set investment goals and to implement an asset allocation based on these goals. As I do this, though, I’m struggling with some emotional stuff. I’ve found that it’s one thing to write about smart investing, but it’s another thing to actually do it.

I’ve just learned a real-life lesson about market timing, for example. In general, short-term market timing doesn’t work — especially for amateur investors. If I asked you to tell me whether the stock market (or an individual stock) will rise or fall next Monday, you’d only be guessing. Investors shouldn’t make decisions based on guesses. Or wishful thinking.

Let me give you an example. I recently decided to sell a large stake in an S&P 500 index fund. In order to get my asset allocation correct, I wanted to transfer the money to bonds. But when it actually came time to sell the mutual fund, I couldn’t pull the trigger.

“What if it goes up?” I kept thinking. The market has been climbing over the past few months, and the fund was up 35% since March. 35%!! That’s a pretty good increase, but I wanted more. “Maybe I should wait until the market goes up another three or four percent,” I thought.

I held the index fund for an extra day. Then two. Then three. Each day, the market went down — and my fund followed with it.

“Ouch,” I thought. “I should have sold!” My fund had dropped 5% from the day I first decided to make the move. ”I guess I’d better just sell. Now I’m losing money that I could have safely on the bond side of my portfolio.”

So I sold.

That was early this week. As soon as I sold, the the market began to rise again. Up half a percent on one day, and the next, and then two percent yesterday.

“Holy cats!” I thought. “It’s up three percent since I sold it. I should have held on!”

This, my friends, is the problem with market timing. You can’t know what the market is going to do from day-to-day. Over the long term, the stock market has returned an average of about 10% per year. But that’s the long term. Over shorter spans, the market is volatile. It swings up and down. Over a period of days, its movements are basically random, unpredictable.

I made the decision to sell on June 12th, but I didn’t pull the trigger until June 22nd. In those ten days, my fund lost over 5% of its value. Now, in the three days since I’ve sold the fund, it’s risen 3%. Obviously, I managed to just about nail a worst-case scenario.

Market timing doesn’t always yield such poor results. But, in general, you’re better off basing decisions on your long-term goals and the market’s broad performance instead of trying to guess what your stock or mutual fund will do tomorrow.