I'm an A cup and always have been, apart from a brief and glorious B while breastfeeding.
At 36, I realise that being flat is BRILLIANT. Clothes look much more chic and there is no risk of ever looking accidentally (or, unfortunately, intentionally!) slutty.
I can be flat as a board in a sports bra or have a bit of shape with a lightly moulded bra. M&S teen bras fit me perfectly and don't gape, but give me a bit of a silhouette.
It could be just the style I prefer - I love Whistles and Cos (and aspire to Celine) and these are all brands that favour the flat of bosom, or maybe I have just learned to dress for my chest.
Also, they don't sag and probably never will - there isn't enough for gravity to be bothered with me. They're still as upright as they were at 20 (although softer now from breastfeeding), and it's a rare D cup who can aspire to that, right?
That's just my experience. I'm sorry that you're not happy with yours, but honestly - women with very large breasts always seem to be complaining about them, and they're definitely harder to dress, so perhaps we should count our (tiny) blessings?