300 calories a day is too little, I know what its like wanting to lose weight, but I can tell you, however hard you think it is at the moment, it will get harder, and then harder again, and there is only so much you can take before you give it all up for some respite.
The difference between eating 300 calories a day and 600 is almost nothing, thats an extra 1400 calories you're consuming a week, which isn't even half a pound, if you eat 900 calories a day, thats 4200 calories a week extra from what you're eating now, which is only 1 and a bit lbs of fat, considering you must be losing close to 5lbs of weight a week on your current diet(am I right?) then all you're doing is slowing your weight loss by 1lbs a week, but at least you're likely to be kept reasonable sane and will make it a bit more sustainable.