Past perfect tenses are used to:
... expresse the idea that something occurred before another action in the past. It can also show that something happened before a specific time in the past.
... we use the Past Perfect to show that something started in the past and continued up until another action in the past (with certain verbs).
But I study hard now is expressing something in present, not the past. You are expressing a present and past action, not two past actions. So I think the simple past tense should be used.
Mostly, when you say "I hadn't studied" - it seems to leave an open question - "hadn't studied for ... some test that happened? What?"
Note that I study hard now says that you are studying hard in general, not for anything specific (you would generally use present continuous if you were - i.e. I am studying hard now for this test or similar), which also makes it sound like you aren't talking about an event but just a habit or characteristic of yourself.