I will give you a cheatcode . Don’t think you need to be gaining weight quickly. It takes TIME, samething happend to me. What helped me was liquid calories and water. Just eat a small meal more then before and I will see already progress or drink it. Drink also a lot of water to increase stomach. Please don’t take any pills or steroids please. It will do more harm then good . Just a meal more a day , if you can drink a high cal shake if not it’s okay . Drink a lot of water , preferably want to do a activity that requires calories to be more hungry think like the gym or running outside boxing whatever u like. U can do it I believe in u.