Only being 19 I wouldn't worry to much about your skin stretching back. I think you will be fine. The only thing is that rapid weight loss is not really a good thing for the skin, so try to consistently lose weight over a decent period of time. 2 to 3 pounds a week is really the best and one of the best thing I can recommend is exercise. I am currently resistance training just to tone muscle, but it gives the skin more to cover instead of just leaving the skin there to hang. I am not bulking out or looking like a weight lifter just toning up, and of course drink loads of water it is great for the skin!