Work out the cal breakdown- it varies so much- it depends on the brand of sauce, the type/amount of pasta, is anything added?
You really do sound like you're in starvation mode- ask yourself- 'Do I want to live the rest of my life surviving on 850cals a day'- thats ridiculous! You really need to try & log your cals for a while- and I mean everything (milk in your tea/coffee, every 1/2 biscuit etc.) to see where you need to work on- i.e. do you need to bump up your breakfast/lunch/dinner, do you need to add some low-cal/fat snacks to bump up your cals etc.
Like me & many others, you've probably heard 'Eat less to lose more' all your life in order to lose weight- unfortunately thats why many of us are life-long dieters- we restrict, lose, then re-gain, then restrict, lose, re-gain, it's a never ending cycle! I'm working on upping my maintenence calories at the minute- I was like you- surviving on 800cals a day or so, but although I lost at first, it soon stopped, and I looked like cr*p (skin/hair/nails looked awful!) so I slowly (100-200cals a week) upped and I'm now on 1400cals a day, but my goal is 1700-1800. I re-gained some of my lost weight, but have maintained this last 2wks so I'm hoping I've turned the corner.
You are going to make yourself ill eating so little- thats the upshot. I know you want to lose weight, but surely you want to be healthy about it? Plus, if you do it right- it will stay off because you'll have re-educated your eating habits & know what you can/can't eat, plus have off-days every so often! You can do it!