Advice for when you're not feeling hungry?

Does anyone have any advice for when you're not feeling hungry at all but probably need to eat? I tend to go through phases where I'm always hungry or never hungry. I've been able to deal with it when I'm always hungry by having some kind of flavored drink, chewing gum, or snacking on something healthy like carrots. Right now I'm in a never hungry phase. I make sure that I eat something for breakfast (normally just a fast food sandwich) and I have something for lunch, but beyond that I really don't eat much. And what I eat for lunch is only about a half meal, not a full one. I know that logically, I probably need to eat more than a breakfast sandwich and half of a lunch. But I just can't seem to force myself to eat anything. I can't even drink anything without gagging. This is definitely something I want to bring up with my doctor, but I'm not sure what this would fall under. And this isn't caused by any medication. I've had this issue my entire life.