。
She's clever, so she already understood what you were trying to say.
Literal
She [topic-は] clever [because-ので] you [possessive-の] want-to-say things [topic-は] was-understanding [you-know-よ].
Two grammar threads share the line. First, ~ので gives a slightly softer, more objective 'because' than ~から — useful when you're stating a reason as a matter of fact rather than personal justification. Second, 言いたい事 is a relative clause modifying 事 — literally 'things-(that-you)-want-to-say'; the listener is the implied subject of 言いたい, recoverable from 君の (where の serves as the in-clause subject marker, a normal switch in relative clauses). The form わかっていた combines ~ている (here a resulting state — 'had grasped, was in the state of knowing') with the past tense, neatly capturing 'she had figured it out already.' The final よ adds a little 'I'm letting you in on this' assertiveness.