。
I'm here for you, so don't worry.
Literal
I [subject-が] attached-am [because-から] worry don't-do.
ついている literally 'is attached' = 'is present/with you, has your back.' A reassuring idiom often used by parents, partners, or mentors to promise support. The negative imperative 心配するな ('don't worry') caps the reassurance. ~から ('because') links the reassuring premise to the directive.