Before pregnancy, I was proud of my firm, b-cup breasts. Now they are saggy and gross. What’s happening? Will they ever be the same again?

Tissue is damaged. What makes young girls have those high shapely breasts is the amount of collagen in their skin and the amount of gravity they've been exposed to. When your breasts expand during pregnancy and lactation, the collagen is stretched and potentially damaged and there is more gravitational pull on the larger breasts. Invest in some really good bras and you will look just fine.
Change is inevitable. Even without a pregnancy, time and gravity will always win in the end. Pregnancy and nursing will nearly always affect the breast size and shape. After delivery, and weaning the baby if you breastfeed, your breasts will return to their non-pregnant state. How closely this approaches their original condition varies due to genetics, weight changes, and luck. Be proud of yourself no matter what!