Having hard time eating calorie intake

Hey, if you weigh 300 lbs, Considering an average 5 ft 10 inches, 20 something years female, 2200 is your BMR. (BMR means the calories your body burns even if you do absolutely nothing throughout the day). You have to eat less than that or burn some calories to lose weight. You don't have to go as far down as 1000. If you cannot maintain it, you can eat even 2000 calories and do a decent workout and burn 150 or so calories.

People around that weight range are highly sensitive to even small calorie deficits and exercises of even mild intensity.

As long as you eat the right amount of protein and exercise, you will not lose muscle. I don't know why people always assume that calorie deficits and cardio causes muscle loss. It doesn't. I have been doing cardio on a calorie deficit for 2 years now and I have gained muscle. I have not done any weight training. Only recently ( a month or so ago) I have added body weight training and plyometric HIIT to my routine. But all throughout 2 years, I have gained muscle. The secret is that you have to have a higher protein intake.

I will give you an example of my one day menu

Morning
whole grain toast (2) with one slice of cheese
250 ml of skimmed milk.

Mid morning snack
5 soaked almonds

Lunch
1 whole egg+ 2 egg whites with hummus

Evening snack
Soy milk 100 ml
80% protein powder
vanilla essence
some coffee grounds
400 ml of water
combine all and make a nice shake

Dinner
lots of water
spices
1/4 th cup chopped tofu
with any one of the following combinations

Cabbage and cauliflower (1 cup each)
or
tomatoes and pumpkin ( 1cup each chopped)
or
mushrooms and spinach (1.5 cups each)
or
basically any combination of any low cal fibre rich veggie you can find.

After dinner snack
1 apple
or
1 guava etc


The whole day adds up to something like 1200-1300 calories. My BMR is right now at 1600. It has high concentration of protein, is vegetarian and very filling for me.
Maybe you can try this too.

I think you are consuming a lot of hidden calories and calories that you are not adding up. eg. oil, butter, milk fat, cream, little snacks here and there. a gummy bear here, a mint there etc.

Make it a habit of measuring your food in either cups, tbsps etc or weigh them and enter them diligently into MFP food diary. Even if you eat a single gummy bear, enter that, If you use 1 tsp of oil, enter that. Enter everything. From what you have told me, I assume that you are grossly underestimating your calorie intake.
If even after honestly and meticulously entering everything for at least a week, you find that your calorie intake is as low as 1000 for each and every day and you have been eating that and maintaining your weight at 300 lbs, you should definitely approach your physician. They might conduct some tests to check your metabolism and hormones. If any medicines are needed, they might give those. If you find that those are normal, too, then adding exercise to your routine will boost your metabolism to some extent.

Also note that with every 10 lbs you lose, your BMR will shift and become lower. A smaller body needs less energy to sustain. You have already lost 21 lbs. That means that when you started (I am assuming at 321 lbs), your BMR was higher than 2200. Maybe at 2300 or 2400. You might have been consuming around 2200 calories per day. This is why you lost 21 lbs but now that you BMR itself has reduced to 2200, you are struggling to lose weight.

Trust me, you will not lose muscle mass. The body will always burn fat first. When there is a deficit of fat or you eat absolutely nothing for more than 24 hours, only then muscle is metabolised.
As long as you maintain above 1000 calorie intake and eat at least 40-50% of your diet as protein, there is no way that you will lose muscle.