this could go VERY POORLY so use at your own discretion, but maybe explaining your attraction to her has only increased since her weight gain could be affirming to her. she might feel insecure because we as women are taught to feel that way when we’re not as thin as possible. it’s good to remind her that she’s also not a teenager anymore and that the weight gain will happen regardless. you’re not children in love anymore. you’re adults in love, and that comes with changes and that’s ok!
all girls want is for you to explain what you see when you see her. like, send her photos of a sunset being like 'this u' (im dead serious), say what you see when you look st her, talk about what you love about her personality. or tell her that you'll support her in everything but that you like how she looks no matter what she decides.