Struggling ...need help!

You're right that being short means you need fewer calories. But the 1200 calorie number came about because it is difficult to get the nutrients you need with fewer calories a day, not because you magically go into starvation mode below 1200 regardless of what your actual requirements are. Most of the daily nutrient recommendations, like vitamin requirements, are independent of your size.

For a sedentary 21 year old your size:

Your BMR value is 1284. This means that your body will burn 1284 calories each day if you engage in no activity for the entire day.
Your daily calorie requirement is 1541 calories.
These calculations are for a female of 21 years of age with the height, weight and lifestyle you specified.

Age is a very important part of the calculation. So you can indeed drop below 1200 as you age. That doesn't mean you have no place to go between starvation mode and weight gain. It means you'll have to make every calorie count by eating a nutrient dense diet, or else that you'll have to exercise. For a sedentary 90 year old woman your size

Your BMR value is 961. This means that your body will burn 961 calories each day if you engage in no activity for the entire day.
Your daily calorie requirement is 1153 calories.
These calculations are for a female of 90 years of age with the height, weight and lifestyle you specified.

Add activity 5 days a week at age 90:

Your BMR value is 961. This means that your body will burn 961 calories each day if you engage in no activity for the entire day.
Your daily calorie requirement is 1490 calories.
These calculations are for a female of 90 years of age with the height, weight and lifestyle you specified.,

Obviously, if you're going to be that active at age 90, it's not a great strategy to be sedentary at age 21