When you're just eating 1500 calories a day, for weeks at a time, your metabolism will adjust to that. You will digest your food even slower, therefore you will need less and less food to keep loosing weight. That is why you're not loosing any weight. Since eating even less food is not really an option, I suggest the exact opposite: eat more food, but of the right kind. Get back to around 2000 calories a day. Initially you will gain some weight, but that doesn't matter. Keep eating around 2000 calories a day, and increase your cardio (imo, less yoga, but that's up to you). Normally I'd recommend HIIT, but since I don't know your current weight, how long you're doing doing cardio already etc, I just might send you to bed with an injury. Nobody wants that, eh?
Recommendation: Buy the book 'Burn the Fat, Feed the Muscle' by Tom Venuto. It's an excellent read on how to loose fat.