I’m 5’3” and 105 lbs. I do barre 2-3 times a week, cardio twice a week, and go on long walks with my dog everyday. I have oatmeal for breakfast, sandwich/salad for lunch, and salmon/chicken with rice and veggies for dinner most days. I also snack on popcorn, yogurt, candy, etc. throughout the day.

I’m in my early 30s.

I am like my older son — I eat about 1300 calories per day and if I go over that, I will gain weight. I think there is a genetic component to our metabolism.

What's your height and age? 1300 calories is something that most people on this forum have hard time achieving in a long run.
But if you really eat only about 1300 calories 90% of the time, there is no way you're gaining weight.