I don't think it ever becomes second nature. It's something you have to focus on all the time, particularly when "triggering" events happen - periods of stress, anxiety, boredom, peer pressure... And most of us don't manage to maintain that focus when we're distracted by life.
I just was talking to a friend of mine yesterday, a psychotherapist, and I asked him how his diet was going. He said, "Oh, I can't deal with that right now ...", and he went on to tell me about all the stress he's dealing with. I don't blame him, but it did occur to me that, even if he had already dropped the weight, if his focus would shift whenever life got stressful or other things took priority, he'd be gaining that weight right back again. And I know what that's like - probably most of us do.
I'm great at losing weight - I've probably lost more than a thousand pounds over the course of my lifetime. Keeping diet a priority and not letting that "lifestyle change" slip back to that "lifestyle same-old, same-old", after the scale isn't moving anymore and I'm at a good weight... not so much.