I've had poor appetite, or I'd frequently lose my appetite after a few bites of my meal for my entire life. I also frequently feel nausea and vomiting is a common occurrence for me. However, over the past few years, I finally learned how to prevent it from happening and I'm able to quickly remedy any nausea issues.
I'm not sure how specific this remedy is to myself, but I found that it was mostly due to nutritional balance issues. If I don't eat the right foods and throw my body's nutrition off balance, my appetite starts going bad and it becomes difficult to hold my food down. I find that whole grains are very important for me to take my food down easily. Whole grains or carbs increases blood tryptophan concentration that stimulates serotonin production that promotes rest and relaxation. Too high of a protein/carb ratio would increase concentration of other amino acids that's not tryptophan, and thus lowering serotonin production. This can also increase amino acids that are precursors of adrenergics, and thus producing the tense, low appetite, nausea feeling that's associated with a sympathetic nervous system stimulation.
If all of that explanation was too technical, a practical plan I follow is I would try to eat a good even balance of all of the food groups between grains, veggies, and meat. If I start feeling nauseous, usually whole grains, such as brown rice or wheat bread, or some snacks such as ice cream will calm me down. Some dessert after a meal is often very helpful, or some fruits to stimulate relaxation.
Also I found that often when I lack appetite and feel like vomiting, that actually means I'm hungry. Try to think of something you crave the most and force yourself to eat. You might at first run into foods that you can't hold down. But gradually, as you try different foods, you'll get a sense of what your body needs, and that food will make you feel really good.
I used to feel stressed out, anxious, and have negative thoughts all of the time. Now that I eat right, I never feel stressed, anxious, or have negative thoughts. Even if I feel the pressure from procrastinating for an exam, if I eat right, I'll just feel really good and won't feel the anxiety.