Hi, and thanks for your medical question on eHealth forum!
Commonest cause of chronic nausea is acidity. Take omeprazole and antacid gel and see if it helps. Liver and kidney dysfunction is another cause. Hence kidney and liver function tests should be done.
Other causes are diabetes and hypothyroidism. Diabetes is often associated with H pylori infection and this also causes nausea and acidity like pain in upper stomach. Electrolyte imbalance too can cause nausea.
IBS, gluten or lactose intolerance, parasites in gut too can be the cause. A stool test should be done for worms, giardiasis etc. Also if nothing is found then food challenge test to detect food intolerance should be done.
Please consult your doctor regarding these possibilities as a confirmed diagnosis to your problem and treatment for it cannot be prescribed on net without examining you. Hope this helps. Take care!