Hi Carly, and welcome to the community!
The people above me^ are on the right track with her advice for you-- as you age, losing weight becomes much harder, and the standard "calories-in-calories-out" model may not necessarily work anymore. This is because the body's hormones and metabolism change greatly as we age. The way hormones work is pretty complicated, so I'll boil it down for you the best I can:
There are two key hormones that see a decrease in activity as we age: thyroid hormone, and insulin. Thyroid hormone controls the amount of energy you consume while at rest (your basal metabolic rate), calorie burn for maintenance of body temperature, and the use of glucose and fat for energy, among other functions. Insulin increases storage of glycogen (stored carbohydrates) in the liver, prevents breakdown of stored glycogen, and lowers blood glucose levels. Because both of these hormones are slowing down, it means that your body is not burning calories the same way that it did when you were younger. You don't burn as much at rest, and because you have less insulin, your body's blood sugar will tend to spike and then rapidly decline, which can lead to weight gain.
There are a few things that you can do to manage the way your body is responding to the food you put into it, which is going to be the primary obstructive factor for your weight loss at this point. This is my best advice for you:
Eat as if you're diabetic.
People with diabetes (many of whom, not coincidentally, are older individuals) have the same insulin insensitivity as elderly individuals, but on a much grander scale. When they eat foods that are high in carbohydrates and fats, their blood sugar will spike, because their cells aren't sensitive to the effects of insulin. In the same way, your body is releasing insulin more slowly, which means that your blood sugar will spike and stay high for longer. Normally, sugar in the blood will be broken down for energy, but when there's too much of it, the body must store it in the cells as fat. When your blood sugar spikes and stays high, the body has to scramble to get rid of it and store it in fat cells until insulin arrives to store it as glycogen in the liver.
The best thing you can do right now is to limit your simple sugars (white breads, pastas, white rice, sodas, candies, etc...) and increase your complex carbohydrates and fiber. That means eating less bread and more vegetables. Even some fruits should be avoided, because they have a lot of simple sugars in them. You can look up "low glycemic index foods" to find out what kind of foods are good for keeping your blood sugar spikes to a minimum. Of course, we're mostly dealing with sugar here, so feel free to eat more protein and healthy fats like olive oil and avocados.
You should also be mindful of the times that you're eating. You don't want your blood sugar to get too low, because when you eat again, your blood sugar spike will be even worse. Don't just eat breakfast and then skip meals for the rest of the day, and make sure that you're eating enough at meals. Nine-hundred calories a day is definitely not enough, and it's just going to make your blood sugar problem worse.
Good luck with your weightloss journey, and hope that this advice is helpful for you.
