In order to burn and keep fat off, it is important to have a
high metabolic rate. Unfortunately, we tend to reduce our calorie intake so low, and increase our exercises that or metabolic rate falls.
Other factors may also be affecting your ability to burn fat. You haven't provided too much detail in your post. E.g. what are you eating? 1500 calories, but what are your meals like? you say regular, but how regular? once a week, twice, three, four times?
E.g.