The medication which you're taking is apt for the condition mentioned. However, gastritis is a major side effect of SSRIs like Fluoxetine. You will have to see your doctor again for the treatment of this side effect and if not improving, may warrant a change of drug. Also, medications are not the only mode of treatment. Combination of SSRIs with therapy has yielded better results than medications alone.
Next Steps
Please consult your Psychiatrist as told above. A Psychologist would also be of help.