Why do my ......?

why do my s droop and have stretch marks? im only 18, so i dont understand why they do, before i thought it was normal but my friend told me that he felt a girls that were bigger than mine and they were up and didnt droop at all, he also said they were firm, im a size 34e, and she is a 36ee, is there anyway i can make my s less droopy, i go the gym every other day, so i dont get why my are so abnormal, i feel really self-consious about being naked with anyone now, i dont droopy s, please someone help!