Gaining weight , I need an ass .

So I’m 30 5”2 and I weigh 80lbs . No matter what I do , I can’t keep weight on . I’ve been seeing drs to see if it’s something medically wrong and no answers so far . I really am so insecure . I have no boobs , seriously only nipples and my ass goes in and has dents ... on my butt cheeks , not the hip dips ( have those too ) anyone have tips on gaining weight and getting a booty . I want to be able to wear normal baiting suits and not have to wear shorts at the beach , and I want to be able to wear sexy underwear. But it all looks crappy .

Edit :: I’ve been to four different drs , I’ve had my thyroid checked and the full thyroid panel thing done like 6 times . I will deff try some of the suggestions... anyone have any brands that actually taste good and not all chalky and bleh lol. The couple I’ve tried were terrible !!!

Another edit :: I’ve been on birth control for a year now due to endo . And that hasn’t helped either .