It's unfortunate, but right now she's heavily countered against the tank meta. From what I understand, Sona does indeed pack a large punch (she has insane poke damage early, and building her AP rewards you with a lot of damage late game). Riot buffering her further could make more Sona players happy, but she'd more than likely be re-nerfed again when the meta decides to change.
I'm not disregarding your misfortune, because it really does suck as a support player (I used to personally main Sona too, but Braum is here!) to have your champion picked off of the meta. I'd personally recommend maybe playing her in a different role for the time being, like mid/top lane. Sure, her wave-clear may be one of the worse in the game, but she could out-poke most of her lane opponents out of farming early.
If you still wish to support with her, maybe try changing up your build-path too. There's no shame in building early damage/magic pen on a support, so long as you're actually supporting your team. I hope that you have better luck with your main. :)