Do you want to start gaining weight now?
Weight gain depends on what causes the body to become thinner: if it is caused by genetic factors, that is to say, if your parents, including your great-parents, are thin, it is not easy to gain weight. You mainly eat a balanced diet and ensure adequate nutrition, as long as you have physical strength and vitality.
If it is caused by your own partial eclipse, then change your partial eclipse and try to get involved in all kinds of delicious and simple foods.
If you have psychological problems, you should change your understanding of the world and the way you interact with people. As the saying goes, if you are broad-minded, you will get fat. If you are always preoccupied and depressed, it is impossible to eat fat.