Who told you to do an 800kcal diet? They're probably not a good source for nutritional information.
If your body had its way, then lying in bed sleeping for 24 hours, you'd burn about 1500kcal (about twice what you're currently consuming). Your body likes to move - it's in your body's design. So if your body had its way, it would probably be burning through 2000kcal each day, maybe more.
Now, it is a known fact that weight gain/maintenance/loss is fundamentally a balancing act of having energy in less than, equal to or more than energy out. So right now you're probably thinking: "Well I'm eating no more than 800kcal a day, that's way less than the 2,000kcal this guy thinks my body needs, so I should be losing weight."
Here's the thing. Your body likes to survive. It's good at this. So when it notices that it's receiving significantly less than what it needs to maintain itself, it politely says "HOLY **** SOMETHING'S WRONG!!!" and it responds in turn by entering into starvation mode. See, YOU know that another meal is coming. YOU know that there's plenty more food in the fridge, and if that food runs out you can just go to the shops and get more. Your BODY does not know this. Your body was not designed by God or any evolutionary process to sit back and relax, knowing that food's at arm's reach. So when your body isn't receiving enough food, it assumes that food isn't there. To survive, it boosts lipogensis and reduces lipolysis; in English, where energy and nutrients were once spread out throughout the cells of the body, now the fat cells are demanding as great a share of the energy received as possible, and they're suddenly very reluctant to let go of any energy.
So, unless you've got some very rare metabolic disorder (in which case the normal rules don't apply), you should consume within 500kcal of what your body needs for maintenance every day. So, realistically, about 2,000kcal every day and regular, quality physical training will probably help you out more than this extremely low-cal diet you're on.
