I don't know why you feel like this, because that is a big question indeed, but I do know someone who has similar problems. He has sensory issues and has Asperger's Syndrome. I suspect that many people with eating problems, especially picky eaters, may fall on the autistic spectrum.
Maybe it is worth looking into it? Maybe a kind of phobia is involved here too, where you become sensitised to certain things. I do not know if treatment for phobia could help you, but again something to investigate perhaps.
I am assuming you have been checked out physically and are not suffering from nutritional deficiencies. These could affect your health generally and may contribute to feeling nauseous.
Who knows why? Who knows why you have ADHD? But you do, and part of having that is learning how not to let it direct your life. Are in any kind of counseling to help you learn to do that?