Weight question PLEASE HELP 😩

Every since I was younger I’ve NEVER been able to keep weight on me it’s hard to even stay 99 lbs ! I’m EBF my now 5 month old I have a 3 year old as well . Does anyone have ANY tips or suggestion to help me gain weight and keep it ! ? pregnancy is the only thing that’s ever helped me gain weight but goes away as soon as baby drops .. please don’t say eat more frequent orrr healthier, or have my thyroid checked , or iron checked I’ve had alllll that done an nothing helps !

PLEASE ANY ADVICE I just want to be a healthy 120-125 lbs I’m 23 and 5’7”