Plain water can be hard for your body to do anything with especially if it's cold and your body has to warm it up first. But no salts or sugars can mean it's hard for your body to absorb it. You really should attempt some fiber or protein as well and give your stomach acid something to work on.
OH.MY.GOD!!! I thought I was crazy because I don't know anyone else that actually hates eating breakfast like I do. It's like a mild nausea, and even the smell of breakfast food makes me repulsed from associating it with eating in the morning. I had no idea it was related to ADHD. Wowwwwwwwww 😮