No. You're killing yourself. You need to calculate your basal metabolic rate in order to find out how many calories you body needs to LIVE ON (meaning breathe, walk, etc) and 750 isn't enough for ANYONE. Women go as low as 1200 and men at 1800. 750 puts your body into a starvation mode which forces you to retain your fat and eat your muscle, giving you the exact opposite to what you want to achieve. If you go below your basal metabolic rate, you SAFELY burn calories that add up each week to either 1 or 2 pounds (2 in your case). Basically, someone here will provide you with a calculator or you could google it for BMR and see how many cals you could safely consume. If you burn 900 cals with cardio then you're fine. Simply don't go over that amount of cals (Your BMR). If you burn, say, 400 cals with cardio and aren't satisfied you could cut down 500 cals from your diet MAXIMUM. Understand? MAX. Since you are 16 you need to be extremely careful. As of now, at 750 cals a day and burning 900 with cardio - you are on your way to anemia, potentially future eating disorder and a variety of other colorful diseases. Worst case scenario in anyone's case is death. The results will come quick enough. Either you have patience or suffer in the end, and if you think you won't (with your routine), we have all been through it. You will. At 40lbs overweight I am *guessing* your BMR is between 1500 - 2000.
Just calculate it and never, ever go below 1200. 1200 should be a bad day for you.