concerning除了可以用作介词外，还可以是分词形容词。

《韦氏词典》

concerning

 adjective

Definition of concerning (Entry 2 of 2)

: causing concern or worry : creating reason for concern : TROUBLING

found the latest reports very concerning

"It's extremely concerning that drivers still choose to give their attention to things other than the road when they're behind the wheel," said [Texas Department of Transportation] Executive Director James Bass.

你的句子中，what's concerning为表语从句，意为令人担忧的事。

I'm focused on 似应改为：

 my mind/attention/effort is focused on 或者

I focus on