I want to add to this discussion ... I know I never weighed 300lbs, but I was pretty close.
When I first started to lose weight I had decided to try the 1200 calories a day thing too, and it backfired on me. My body could in no way survive off that mere 1200 calories and I became ravenous and all I could do was gorge myself on food. I ended up gaining the weight that initially had just melted off.
Instead of limiting your calories so much why not look at creating an ideal calorie deficit. What I did is calculate how many calories I burned a day just by being me, and then tried to maintain a 1,500 deficit a day. Meaning that if I burned say, 3,500 calories that day, I aimed to eat 2,000 calories. FitDay is a great website that you can use to help you keep track.
Even now, when I eat a low amount of calories [specifically 1,200 or under - which does occasionally happen] there are ill effects. To start with, I become VERY HUNGRY and it's very hard to resist the temptation of eating everything in sight, I also become irritable, and very tired. If I go to sleep like that, the next day if I work out as well I usually get sick. My body NEEDS more calories than that to function.
Experiment with what works for you, don't forget that this is a lifelong journey and you'll find exactly what works best for you.
