Yea I definitely let myself go. I really don't understand how some fighters also let themselves go for like a year then train for 3 months and go back to competing professionally.
A big part is the sport's focus. The milestone is the prize that is the gold, and once its achieved, drive and motivation is really out the window. It happens alot here in the ammies, coaches and the community will build up a certain tournament or title, and people will grind day and night to get that, once they do, they feel there's nothing left in the sport and check out after. For the most part, it's pretty short term. Sure, there's always examples of people who do practice and continue to train in their 50s, but really, they're the exception, not the rule. Burnouts are pretty common, its no different than any competitive industry.
One thing TMAs have thats better in this regard, is people who join those gyms do it for the lifestyle or the community aspect, and tend to keep training long term. I've had close teammates who were quite talented just quit fitness in general after achieving their short-term goals. A combination of focusing on their real careers, families, and other hobbies ended up soaking up their time. A couple have gotten burnt out so bad, they quit combat sports; One basically was forced into an unnatural weight class, and was expected to maintain that weight in the event a fight appeared, and the injury rate started to pop up more frequently; Shortly after, he just called it quits. Training twice a day 6 days a week, hard dieting with no real break does that. It started interfering with his personal life too. Been there, so I get that part.