saggy and heavy breast happen when the breast isn't supported well that is when you wear a loose bra. To correct this start wearing a well fitted bra and your breast should be well supported and lifted from the chest wall. that's how you can prevent further sagging. To correct the saginess which is already present you can try yoga specifically for this purpose. surgeries for breast reduction is not advisable as you are unmarried and haven't completed your family. I would suggest you a well fitted bra with a bigger cup size and yoga to increase the tone of your breast.