Diet With Too Few Calories Unintentually????

I went on a low calorie,low fat,high protein diet in order to lose some weight. I read 1800 calories is the min
I had Orange Juice for breakfast…. 190 calories
Roast Beef with light Mayonnaise …..550 calories
Hamburger with Ketchup and bread…400 calories
To drink I had either water or diet soda….Both 0 calories.
That means I am only eating 1140 calories a day…..That seems too small…. The minimum recommended is 1800….But It just seems like that is overindulging
Any suggestions on the diet or even exercises?