I know, but there's nothing I can do about it.

Literal

Understand [concessive-んだけど], can't-help-it [explanatory-のよ].

分かってるんだけど is 'I understand, but...' — ている contracted to てる, plus the explanatory ん (contraction of の) and the concessive けど. どうしようもない means 'there's nothing that can be done' — a useful set expression. のよ at the end adds feminine emphasis.