How to find a bike that fits

Today I identified the scariest part about deciding to do a triathlon for the first time.
And no. It's not potentially drowning.
The scariest part about triathlons is buying a bike.
Seriously.
Bikes are expensive. And complicated.
I don't know anything about bikes.
I've purchased bikes. I've ridden bikes. And I can change and pump up the tires. But that's about where it ends.
So when I bought my $1100 Specialized Comp brand new from a dude on Craigslist I admittedly didn't know quite enough about it.
I had a 52" women's Trek in Chattanooga where I learned to ride on the road. And it had always felt too too small. So I figured a 54" frame would be fine. That's logical, right?
Ash Beal at the Sports Basement in the Presidio set me straight. He took one, long look at me holding my bike when I walked in for my fitting and said, "There's no way I'm going to be able to fit you on that bike."
I just stood there. Slightly shocked.
Then he said: "You have really short legs."
Man.
It's bad enough he's just told me I spent $1100 on a bike that doesn't fit me. But does he have to insult me, too?
Actually, Ash was fabulous. He took my measurements and tried to advise me on the size of bike I needed.

Sure enough, mine was a good 5-7 cm too tall for me.

I told Ash about the Trek I used to ride and how it always felt too small. He said that was because I have a long torso and the aforementioned short legs. He told me no matter what, I need a men's bike which will have a longer cross bar.
"Anything long and low," Ash said.
And he told me that the most important measurement was from the middle of the seat to the middle of the handlebars. Mine is supposed to be 654 cm.
Basically Ash talked me into selling the bike and trying to get another that fit me better. Ash said I could call him if I found a bike and wanted to know if it would work. And because he couldn't fit me to my bike, he didn't charge me for the fitting. So now I'm off to Craigslist, looking for a 49" men's bike with a 654 cm seat to handle bar measurement. My boyfriend prefers carbon fiber, but at this point, I'll just take something that fits.
My advice: Go get fitted for a bike BEFORE you buy one.