You've got to eat more. More more more. If you don't you'll end up stalling out your metabolism. Plus at 900 calories a day there is no way you are getting enough nutrients or fiber to keep you healthy. If you don't feed your body what it needs, it will cling onto fat for survival - it's way smarter than you are at clinging onto fat.
Here's a concise description of what is happening:
(btw this is written by someone who had an eating disorder - I don't think you do, but it does explain why "low calorie dieting is a bobsled to hell.")
She has a lot of other very informative articles here:
Using this site (http://www.bodybuilding.com/fun/issa64.htm
) I calculate your current maintenance calorie needs to be 2380 per day.
If your ultimate goal weight is 120 pounds, you should eat around 1800 calories a day.
That number may have blown your mind. But your body is a furnace, and in order to keep the metabolism burning, you have to feed it often with the best quality food you can.