You will find many people here that have had the Nissen, including myself. I had it in November, and it has been a life saver for me. I went from being in a horrible place to feeling better than I ever have in my life now. Its been 5 months exactly since my surgery.
Everyone recovers differently, but I think what you heard would be the most extreme, unusual case of recovery. You won't have to re-learn to swallow. I could swallow normally as soon as I woke up. Yes, you do have a restricted diet for 6-8 weeks. In my case, it was clear liquids for a week, thick liquids for a week, soft foods for a week except for meat and bread, then I was basically back to normal. At 6 weeks I was eating everything including meat and bread. You will lose some weight, but I doubt it would be anywhere near 1/3. I am about 235 pounds and I lost 25 in the first 8 weeks. I have since gained it back and added more on, since I can eat anything I want now without consequence.
Hospitals aren't fun, but you won't be there long. I had my surgery in the morning on a Tuesday, and was going home first thing on Wednesday, less than 24 hours. Someone recently on here went home same day as their surgery. You shouldn't expect to be in the hospital long.
How bad is it? In my case, it was easier than I expected. The pain is well controlled with painkillers. In my case, after 10 days, I felt no pain at all. Many people have pain less time than that. Some more. Like I said, there's no re-learning swallowing, you should expect to swallow just fine. By 8 weeks, you'll probably be eating mostly everything. Some people recover much faster. I've seen stories here of people eating bread and meat at 4 weeks, while some take 12.
I'm sure others will chime in with their experiences. You'll find lots of success stories and positivity about the surgery here. It has helped many of us.