。
She always wants to know what I'm doing.
Literal
[topic-は] I [subject-の] do thing [object-を] always wants-to-know.
~たがる is the third-person desire form — Japanese famously distinguishes 'I want' (~たい, an internal feeling only the speaker can directly know) from 'someone else wants' (~たがる, observed from outside). 知りたい would be 'I want to know'; 知りたがる adds the suffix -がる, which roughly means 'show signs of wanting.' This grammatical separation reflects a broader Japanese pattern of treating mental states as private to the speaker. The relative clause 私のすること ('what I do') uses の where in a main clause が would appear.