how do i get my cat to lose weight?

my cat is almost 4 years old and 20lbs. She’s been overweight most of her life. I’m not sure if it’s caused by how we used to feed her but we fed our other cat the exact same way and she isn’t overweight. we’ve been to the vet multiple times for her weight and have had her eating many different kinds of prescription cat food. we’ve measured how much food we’ve given her for 3 years and no matter how little her servings are, she won’t lose any weight. i’m not sure what to do because every time we take her to the vet they just give us a different kind of food and a different amount to give her. she hasn’t lost any weight. she actually gains another pound every year. she’s pretty active and plays outside a lot so she gets enough exercise i think. she has trouble cleaning herself because of her weight and its heartbreaking to see her like that. does anyone have advice to help her lose weight?