Your mother’s good intentions are fucked. She is epitomizing the roll of what we like to call a “concern troll” - someone who uses the “but I’m just worried for your health” excuse in order to blatantly body/fat shame, as if there is an excuse. Of course she loves you and wants what’s best for you, but I can tell you right now - her knowledge of “what’s best” is heartily misinformed.
People of all sizes and weights and levels of health have stretch marks on their bodies. Weight does not determine health and health is so hard to define, changing from person to person.
You should not feel like you have to change yourself for anyone, or for any reason, except your own happiness and wellbeing. Not everyone can grasp this concept. It’s not widely promoted or accepted, but I can tell you right now - it’s 100% correct.
Take stock of how you feel about your body. How much does it have to do with what others are telling you and how much does it have to do with what YOU want for yourself? Why? Determine these things for yourself. Consider your health and what you want to achieve for yourself, without bringing weight into the picture. Instead focus on things that will honestly bring you happiness - tangible things, not grounded in aesthetics or dress sizes.
For example, my health goals at the moment involve improving my flexibility and my general mood/mental state. I stretch daily, work on breathing techniques and meditation. When I feel like being active, I do something I enjoy that I know will bring me happiness and not feel like punishment. Movement should not be about punishment or trying to beat your body into submission. It is so easy to go to that negative place.
When I was your age, I went to that negative place often, because I didn’t know any better. Despite my active lifestyle and relatively normal eating habits, my fat body continued to exist. At the time, I thought it was because I wasn’t working hard enough. In reality, it was because I was perfectly FINE the way I was.
I’d be willing to bet your mother has her own set of personal body image issues she may or may not be projecting onto you. Maybe that’s something you can tackle with her at some point, introduce her to new ways of thinking. For now, please focus on you. Listen to your body, form a relationship with it, know that you know yourself better than ANYONE else.
Try not to let your mother’s comments effect you, though I know from experience it’s likely they will. Try telling yourself: “She’s trying to help, but she doesn’t know. Only I know what’s best for me.”