Hasn’t Shane Mosley been here before?

The three-division champion from Pomona, Calif., was on top of the boxing world in 2002 – both literally and figuratively – when Vernon Forrest knocked him off his pedestal in a huge upset. Strange thing is, he fought (and lost) to Forrest twice, when he probably should have never been in the ring with him in the first place.

Sure, “Sugar Shane” had been dethroned by Forrest in the amateurs, and the “Viper” was a top contender. But Mosley could have gone after bigger fish, for far bigger money.

So why’d he bother with Forrest?

Boxing is a business, after all. It’s about getting the most bang for your buck, not getting revenge. Or proving you’re tougher than everyone else. Outside of the ring, anyway.

It’s a lesson that Mosley has yet to learn, apparently.

Two years later, in 2004, Mosley, coming off an upset win over Oscar De La Hoya, fought another anonymous-but-talented opponent named Winky Wright, in defense of his WBC junior middleweight title. Again, he didn’t have to fight Winky. He had positioned himself as one of boxing’s stars and, at that point, those alphabet titles and mandatory title defenses are about as meaningful as a Hector Camacho Jr. fight.

But Shane opted to fight Wright anyway. And got himself beat. Again. Shut out, in fact.

Like Forrest, he tried Wright again eight months later, and did better. But Mosley lost that one, too.

And, for the second time in two years, Mosley – who had a multi-million dollar showdown with Felix Trinidad signed, sealed and delivered – blew a potential fortune.

Which brings us to Saturday.

Mosley is riding high after two consecutive knockouts of Fernando Vargas last year. Sugar stands to make a career-high payday against the likes of Floyd Mayweather (provided he beats De La Hoya in May), and has other possibilities, if that one doesn’t work out – showdowns with Cory Spinks, Miguel Cotto, Antonio Margarito and, maybe, Ricky Hatton.

So, instead of waiting for Mayweather or another big fight to develop (he was rumored to have turned Mayweather down in November – on the advice of his, ahem, wife), he’ll fight Luis Collazo, a former champion and one of the more dangerous 147-pounders on planet Earth.

For comparative chump change than what he’d get for “Pretty Boy” Floyd.

That’s not all.

Collazo wasn’t Mosley’s first choice. No, his first choice was Kermit Cintron, another tough customer and one of the hardest-punching welterweights in the world.

But even he was a better choice than Collazo, a slick, stick-and-move stylist who can make just about anybody look bad. Just ask Hatton.

So when Cintron dropped out with a dispute, Mosley handpicked Collazo. Not France ’s Frederic Klose, whom the geniuses at the WBC somehow rank No. 3 in the world. Not someone named Alpasium Aguzum of Turkey – ranked No. 6 by the same WBC. And not the well-worn Golden Johnson, No. 13.

No, he went with Collazo. Who will probably beat him.

Somewhere along the way, Mosley began to actually believe that he is a welterweight. And he’s been at 147 pounds (or over) for so long, it’s hard to remember the days when he was a lightweight.

And he was a fabulous 135-pounder. Tremendous. Unbeatable. Lightning-fast, powerful, strong.

When he moved up to welterweight to fight De La Hoya in 2000, it didn’t seem like such a bad idea, since Oscar was himself a natural junior lightweight.

But almost all of Mosley’s losses have been a result of him being a little too short and a little too short-armed. Forrest vs. Mosley was like a latter-day version of Thomas Hearns vs. Pipino Cuevas. The “Viper” was simply too wiry and long for Sugar Shane. Mosley found himself physically overmatched.

Same for Wright, whose endless southpaw jab kept Mosley at bay for 24 rounds.

Collazo? He’s 5-foot-9, just like Mosley. And he’s even got a shorter reach than Sugar Shane at 72 inches.

But those numbers are misleading. Shane possesses a broad, muscular back that makes his arms appear longer than they are. Collazo, meanwhile, is all arms, as he proved against Hatton last year.

Those arms almost forced a never-before-seen Hatton loss.

Beyond the physical ramifications, Collazo is the younger, fresher fighter. He’s a southpaw. And he’s a natural welterweight. It’s almost as if Mosley searches over the toughest fighters in his vicinity, and challenges himself to take the fight – regardless of the future ramifications.

Why Mosley would risk his world ranking, his star power, and the payday of a lifetime for a fight that means next-to-nothing is a mystery.

Why he’s doing it for the third time in three years is even more mystifying than that.

So don’t be surprised to see Mosley dejectedly talking to Larry Merchant Saturday, talking about how he’ll be back, and how it just wasn’t “his night.”

At 35 years of age, however, getting back may be easier said than done.