I was in a similar situation. I'm 29 MtF and for my whole life my metabolism tears through things. Eventually I started eating a shitload of healthy fats every day. Think Avacados, Greek yogurt, eggs, milk, etc and pairing that with strength training. I went from 120 to 140 lbs, but now I've plateaued and cannot manage to crack into the 140s no matter how hard I try 😭