Credit: fit2fat2fit.com

ABC News' Sabina Ghebremedhin reports:

A little more than a year ago, personal trainer Drew Manning stopped working out, and started eating fast food, white bread, sugary cereal and soda.

Manning, a self-proclaimed fitness addict, started the weight-gain journey to better understand what his overweight clients go through.  He let himself go completely, and chronicled the process in video blog on his website, Fit2Fat2Fit.com.

Manning gained 23.5 pounds by week 4. As the weeks progressed, his confidence - and his health - took a downturn.

He started to get winded easily, and his glucose level and blood pressure were high. "It's getting a little scary," he said in one of his video blogs.

Manning's wife, Lynn, was prepared to see the physical changes in her husband, but didn't expect the emotional and personality changes that came with the weight gain.

"His self-confidence, that completely went away and depleted," she said, explaining that he was "becoming lethargic, lazy, not helping around the house."

"I was in denial at first until she kept pointing out the things I was doing," Manning said today on "Good Morning America." "But I did become lazier.  … I had less energy so I did become exhausted and I kept seeing how it affected our relationship because of that. And so that's where the biggest surprise was, the emotional [part].

When Manning started his experiment on May 7, 2011, he had a 34.5-inch waist and 17-inch neck, and he weighed 193 pounds. Six months later, he had a 48-inch waist, 19-inch neck and he weighed 265 pounds. His clothes didn't fit.

Last fall, he ended the experiment and started to whip his body back into shape.

"It was hard to get back into the exercise routine," Manning said today on "GMA," revealing his new physique.  "This is the first time going to the gym that I was nervous. Before I loved going to the gym, but for the first time in my life, I was humbled. … doing push-ups on my knees, doing assisted pull-ups, things like that for the first time, it was a very humbling experience."

Manning tells his amazing story in his new book, "Fit2Fat2Fit: The Unexpected Lessons from Gaining and Losing 75 lbs on Purpose."

Click HERE for Manning's recipe for Farmer's Scrambled Breakfast.

Click HERE for Manning's Thai Turkey Skillet recipe.

Weight-Loss Tips: Avoid These 5 Mistakes

What are the five biggest mistakes that people make when they are trying to lose weight?  Below Drew gives the answers and recommendations for overcoming those obstacles.

1. Lack of flexibility. There need to be scheduled breaks. When there is no flexibility or detours (where every "bad" decision feels like a loss), it'll be impossible to stick with. We all make bad choices sometimes, so we shouldn't beat ourselves up about it.

2. Prepare, prepare, prepare your meals in advance. When you fail to prepare your meals in advance, it becomes so much easier to give into the temptations that are everywhere (fast food/most convenient food option). When meals are prepared in advance, you won't feel that same temptation.

3. Thinking that just because it's low-fat or fat-free it's good for you. A lot of the foods I ate during my Fit2Fat stage were considered low fat (white bread, white pasta, sugary cereals, granola bars, sodas, juices, SpaghettiOs, Ramen noodles, etc.)

4. Not understanding the psychological battles that come with weight loss (plateaus, food addictions, fear of being judged at the gym, etc.). Understanding the physical side of weight loss (diet and exercise) is one thing, but I learned that the toughest part is overcoming the mental and emotional battles.

5. Focusing too much on weight. Weight is not the best measurement for overall health. You can lose weight in an unhealthy way, so just because you're losing weight doesn't mean you're becoming healthier. People need to focus more on becoming medically healthy first rather than just becoming skinny or weighing less.