So many people are so accustomed to eating sugar that they feel bad if they don't get some with nearly every meal. When you're sugar-addicted, not eating it makes you feel crappy in the short-term, but eating it makes you feel fine.
Actually, in my case at least, I wasn't "fine" eating sugar, even though I thought I was. After eliminating sugar and flour, so many big and little health issues resolved that I had no idea in the world were connected to them: the awful migraines went away, the brain fog and lack of motivation disappeared, my hands and feet were finally warm again, I didn't have to run to the bathroom every 1/2 hour to urinate, my energy increased, my moods and emotions became much more even-keel, I no longer felt like everyone was out to get me and that I was all alone in this world (even though I wasn't!), and that overall feeling of doom and gloom just disappeared.
Now, that gloom is the first thing I notice when I get a trace of sugar. I've learned how happy life can be without it, that I now avoid it at all costs, even though people might think I'm being pickier than I need to be. But THEY aren't the ones who have to feel its effects for the next day or more.
So, I guess I'd say that it's best to avoid it completely if at all possible during a trial run. But I do remember that when I stopped eating the obvious sugary foods AND stopped dairy, too, that I felt better almost right away. It was only after that, when the above symptoms came back, that I'd stop and say, now WHERE did I get flour or sugar and then discovered where the trace amounts were lurking.
Also keep in mind that mood-related reactions to sugar are often delayed. Mine don't show up until about 12 hours after I eat. So when I get a reaction I look back to what I consumed or encountered 12 hours ago to find the culprit.