cant lose weight for the life of me

what is my problem? ive been trying to go on diets since i was 14 (im 21) but i cant ever go past a week. i even gained at one point, thats what gave me a permanent double chin (i had one before but it was less noticeable), and now im probably never going to lose that weight, apparently. it feels worse because my sister and mom are on Ozempic and they keep telling me im going to gain weight while they lose it, as a “joke”. im like the same weight as this other girl whose a daughter of a family friend, and my mom is so mean about her being fat. she doesn’t do it to me but that doesnt make me feel better. its really the only insecurity of mine i SHOULD be able to easily change but i can never manage. what is wrong with me