Not eating enough calories to support your day-to-day activity levels will easily crash your metabolism, making it nearly impossible to lose weight.
There's a nasty myth circulating in the diet world that (800/1000/1200) calorie per day diets are the key to weight loss.
What's actually happening is that your body is starving and entering fight or flight mode. It's going to hang onto every bit of fat it can so that it has something to survive off of, since it has no idea when you're going to feed it again.
Your body wants to keep you alive. FEED HER/HIM.