How to gain weight

I’m having trouble gaining weight on my own I tried to lose my baby fat from having my son and I just got way to skinny and I’m so insecure about it I use to have a amazing ass but now it’s just there and my boobs are so saggy cuz I went from a ddd to a b cup I’m 21. Years old but look like I’m a granny , I try to eat but I have no appetite even when I smoke weed I eat a lot of fatty food but I can’t eat big servings of me and I can also sometimes just eat once a day with snacking in between because sometimes I’m just not hungry I need help I’m tired of people calling me sickly or a dope head . I’m always tired and my mental health problems just keep getting worse because I hate my body