Jeb had always said to think with your brain, not your emotions. He'd been right, as usual. So I put all my feelings in a box and locked it.
Jeb advised to rely on logic rather than emotions when making decisions. His wisdom proved true, guiding the narrator to think clearly and rationally in difficult situations.
The narrator heeded Jeb's advice, suppressing their feelings by metaphorically placing them in a box and locking it. This reflects their attempt to stay composed and focused through reasoning instead of emotional reactions.