Feel so insecure about my breasts

I am 17 and I have a loving caring boyfriend, but I've become more and more insecure about my breasts. They are quite saggy and the last time I checked I was a 32 DDD, but honestly now my bras are already too big for me and they used to fit perfectly 3 months ago.. now my boobs are getting smaller and saggier, and I'm getting even more insecure about taking my bra off. Is sagging even normal at this age and for my bra size?. I don't have stretch marks on the top of my breasts if that helps.