Help me out here people! I'm feeling damn guilty over a notebook...?

First of all, you simply made a mistake. These things happen from time to time. As long as you didn't read what was in her notebook, you're only guilty of making an honest mistake. If she's unforgiving, that's her problem. Just ignore her gaze and stop trying to apologize; you've already done that. Second of all, and correct me if I'm wrong, but I think that the reason you feel weak and lose all confidence when you apologize is because this confidence is just a mask you put up to hide the weakness you feel inside. It's not a shame to feel weak. All men do at various points in their lives.