When Diego Sanchez thinks about the night he took on UFC lightweight champion B.J. Penn in Memphis, Tenn. – so far the only UFC title fight of Sanchez’s decade-long career – there’s really only one word that comes to mind.

“I look back on the situation, and I’m just more heartbroken than anything,” Sanchez (23-5 MMA, 12-5 UFC), who fights Takanori Gomi (34-8 MMA, 3-3 UFC) at the UFC on FUEL TV 8 event in Tokyo on Saturday, told MMAjunkie.com (www.mmajunkie.com). “That is the only time in my career where I truly got my ass kicked. Not many fighters can say that.”

And that’s true, there aren’t. There also aren’t many fighters who can say that their only UFC loss as a lightweight came against Penn, arguably the greatest lightweight of all time, who was just then right at the peak of his powers. Sanchez can say it.

It’s just that, when he looks at the state of his career today, it’s tough to be sure what simply being able to say all that really counts for.

Consider the situation: Sanchez, who was an undefeated 23-year-old when he became the very first winner “The Ultimate Fighter” back in 2005, is now a 31-year-old veteran coming off a shoulder surgery and a rocky stint as a welterweight. He’s headed back to lightweight in part because, as he put it, “I’ve always been a lightweight; I’ve just fought a bunch of tough welterweights.” But it’s also because, like every other fighter in the UFC, he wants to be a champion. There might have been a time when he thought he could do that at 170 pounds. Now, Sanchez said, he realizes that he’s not any bigger or stronger when he fights up a weight class. All he is is less disciplined, mostly because he can be.

“As a lightweight, I probably could beat myself up at 170,” Sanchez said. “That’s how important that discipline is.”

The thing you have to remember, however, is that these are all fairly recent revelations for Sanchez. So is his realization that, whether he wants to admit it or not, his days as a pro fighter are numbered. Every fighter knows that – intellectually, at least – when he starts his career, but few can get themselves to really believe it at first. Sanchez had eight months following surgery to repair a torn labrum in order to think about it.

“MMA is my baby,” he said. “I got my baby taken away from me for eight months. I was sitting on the couch, and it really made me figure some things out. It really made me realize that I can’t fight forever. My body’s not going to hold up forever.”

It probably doesn’t help that, even for a pro fighter, Sanchez is rough on his body. You look at his stints as a welterweight, and you see some bloody battles, the kind he’ll wear the proof of for the rest of his life. Sanchez has, by his own admission, “a big heart and a strong mind, but a lot of times my mind is stronger than my body. That’s how I end up getting injured.”

Even so, he’s still never been knocked unconscious in a fight, he pointed out. He’s never been knocked unconscious in training. And the only time he’s ever felt like he got thoroughly and absolutely beaten up was that one time, that one title fight, which still stings every time he thinks about it.

Back then, Sanchez said, he was “young and very immature.” He’d left his home in Albuquerque, left Greg Jackson’s gym. He thought he was all grown up and ready to be a champion.

“But I was doing a lot of stuff that I shouldn’t have been doing, living a very different life from the life I live now,” Sanchez said. “I was out on my own, single guy in California, living a bachelor life according to my own rules, my own schedule.”

When he got his shot at Penn’s lightweight title, he thought his time had come. “But I wasn’t at my best,” Sanchez said. “I wasn’t ready. I just wasn’t ready for the big dance.”

What he remembers of the aftermath is pain. Not just the pain that comes with a trained professional putting his shin upside your head, either. We’re talking embarrassment. We’re talking heartbreak.

“I took the wrong road after that,” Sanchez said. “I covered it up with alcohol, with partying. I was trying to cover my pain in the wrong ways.”

He tried to reinvent himself as a welterweight, with mixed results. He ran off a string of “Fight of the Night” performances, but he didn’t even really get close to another title shot. Then came the shoulder surgery, and with it the mandatory time off. Plenty of time to sit around and think about where he’d started and where he’d ended up. There was still time to do something about it, but there wouldn’t be for long.

“The clock is ticking for me as an MMA fighter,” Sanchez said. “I want to realize my dream to become a world champion, and I realize my best chances are at 155 .”

These days his life is different, Sanchez said. He’s a married man, no longer living the bachelor life, and his discipline is better for it. He’s back home in Albuquerque, back at Jackson’s gym and appreciating all the things about a fighter’s life he took for granted before, he said, “the little things that suck,” like getting up sore in the morning and going for a run. He’s also back at lightweight, hoping that a run at the title starts with a win over Gomi in Japan (FUEL TV, 10 p.m. ET).

Is that possible? Is there still time to make sure that, once it’s all over, he’ll have something more to brag about than the fact that he only got his ass kicked once, and by a champion at his peak? Maybe. Sanchez will have to write the rest of that story for himself. At least now he knows that the future isn’t an endless supply of blank pages.

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