My second daughter, for a long time, was the one who challenged me most as a parent. While she wasn't a bad kid (what kid is?), she did things that didn't make sense, long after the age when I thought she would choose better. She woke up cranky some days, disagreed at random openly and loudly, fixated on one idea, was poor at transitions between activities, even fun ones, and didn't like to take any sort of direction.
Always looking to the future, I had several trajectories in mind. On good days I would imagine her as a future CEO, leading her team with uncompromising vision and energy. On bad days, I imagined her doing lines of coke in the bathroom and checking into rehab. (Not that these two are mutually exclusive.) Both of these visions, and my worrywart tendencies in general, weren't helping. It was time to focus more on the present.
Since I had read so much about nutrition changing behavior, and since we already knew that food dye had an effect on her, and since we all have to eat anyway, I decided a few summers ago that it would be a "paleo summer".
Now, I knew we wouldn't adhere to anything perfectly. I also knew that the odds were we wouldn't need to be "perfect" to see results. Kids often recover faster and better than adults, and their behavior is a more direct reflection of their mood. So I set out to do everything I could to conveniently eat grain and dairy free. I cooked with almond flour and coconut milk. I bought even more fruit and vegetables. I made more smoothies.
At friends' houses she would end up having grain or milk, and when we were traveling I just did what I could. Really, she probably ended up having a serving of either grain or dairy every day.
But it still made a huge difference. She went from being the drama queen of the family to an "easy" kid in the course of a couple of months, In fact, the harder thing to change was my own habit of bracing for a struggle about every transition of the day! We haven't ever been quite this dedicated since then, but overall as she's gotten older, she chooses a variety of foods and has learned that more nutritious food makes it easier to get along with the world.
I hope that our story inspires you to try something, even if it won't be the ideal you think it ought to be. Do what you can and see what happens. I learned a lot, and it wasn't hard to switch gears over the summer when my time for shopping and cooking was more flexible. And if you've tried something like this, please share!