One thing that is worth bearing in mind is this.
You will get lots of people recommending eating as you already are but smaller portions- this only works if your diet is already good to begin with, but you eat a lot.
Other people will recommend vlc diets which are unsustainable in the long term.
Other will recommend low fat/high carb does and others will recommend low carb/high fat.
And all of these will work for someone at some point and for an amount of time.
Things to consider.
Your body needs a certain amount of fuel to just function. Even if you never got out of bed, your internal organs, digestive system, cardiovascular system, immune system, endocrine system, brain etc need calories just to function and their requirements will be more than 800 calls a day. That is your BMR.
You shouldn't go lower than that. Your body will be forced to use fat reserves but won't use them very well or efficiently. Your body will become used to surviving on a very low, unsustainable calorie intake and adjust its requirements accordingly. You won't necessarily notice these or recognise them immediately. But that cut on your finger might take longer to heal or that cold might linger for longer than it used to or you might develop heart arrhythmia.
Then, when you start eating 'normally' again, your body has got used to functioning on less so the 'excess' calories in your system will be greater, which is why people put back on the weight and more when they finish dieting.
Carbs (of all kinds) cause your body to release insulin. Protein does too but to a much lesser degree. Fat does not trigger an insulin response.
When there are fewer carbs in your body, your body will switch to burning fat rather than carbs as energy. Your body can't burn fat while there is insulin present. Insulin is a hormone which tells your body to store fat not burn it.
As someone else has said, vlc effectively starves your body of what it needs.