My upper bust is 84 and my lower bust is 68. What size bra should I buy?

According to your upper bust and lower bust, I suggest you buy a 70D or 75D bra. It should be noted that different brands and styles of bras may have different sizes. It is recommended to try on several different models before buying to find the size that suits you best.

In addition, if you are not satisfied with the shape of the chest, you can consider using some adjustable underwear to improve the appearance and shape of the chest. However, it should be noted that the adjustable underwear can not completely change the size of the chest, but only make the chest look fuller and more straight by shaping.

I hope the above information can help you.