I dont know what i should do about my body?

I am 16yrs old, and i have F cup . Im not over weight atall. im average. i did not inherit this size. my mom is such smaller. im finding it difficult to find F cup s? as alot of sizes only go up to DD (double d) im finding it very irritating how when i get properly fitted, the bra always makes my sag. so i buy small bras about b or c cup to actually lift my breats and keep them in place so they do not sag. even tho this size is very small, and it can be painful. but i dont like them saging. i dontknow what to do. i try shortening the straps. but that dosnt work? im just getting irritated. i find small bras lift them, and keep them relitivly in place. my boyfriend loves my soze, i dislike it very much. they are still growing.