maybe she’s just concerned you’re going to get too obsessed with it? eating disorders are common and she’s probably just worried. if that’s not the reason then she might just be worried about the costs. healthy eating doesn’t have to be expensive but if the rest of your family doesn’t eat healthy then it’s like paying 2 grocery bills each time.
start small. maybe eat what your mom cooks for supper but eat healthier during the day? that way you aren’t buying tons of different groceries only you’ll eat. research what healthy eating is and show your mom the information (maybe even get a doctors appointment with your mom and your doctor could give her info). it’s really hard to give an answer to this question since i don’t know the reason why your mom won’t. i know for me when i wanted to eat healthier in the beginning my parents got annoyed because they had to pay two grocery bills basically. i started helping out using some of my own money and then they got over it and even started eating a bit healthier.