800 calories is not enough. The body will always try to adjust to what it is given, (homeostasis) if you won't give it enough food it will respond by lowering your metabolism. At that point you will be starving (and tired) and not losing weight. It (your body) will hold onto your fat for dear life! This will happen at some point; sooner for some, later for others.
I went thru this and my trainer explained to me why I had stopped losing weight despite dropping my calories. I know it seems almost intuitive to us, less calories = more weight loss, not always the case. As soon as I raised my calories to 1200 I started losing again.
I strive to get at least 120 gm protein, 20 to 30 gms fat and the rest in carbs. I choose low glycemic carb sources like whole grains, beans and fruit.