Correct, there is no explicit, or implied, limit to the feat. But shaken can/does become fear when applied more than once. (More below)
Mechanically: Shaken last 10 rounds, renewing it each round is a bad idea (sea below)
Fluff : Anxiety over further dishonoring yourself. (More below)
Benefit: As an immediate action, you can add a +1 bonus to the result of an ability check, attack roll, saving throw, or skill check you have just rolled by becoming shaken for 1 minute. You must choose before the result is revealed.
(Reminder : You are shaken for one minute after using this. Combat rounds are 6 seconds, you are shaken for 10 rounds after using this.)
- Technically becoming shaken while still shaken will make you fearful. (Or, at least make you save vs being fearful.
"Becoming Even more Fearful" says: (PF Core p.563)
Fear effects are cumulative. A shaken character who is made shaken again becomes frightened, and a shaken character who is made frightened becomes panicked instead. A frightened character who is made shaken or frightened becomes panicked instead.
Now, to answer... this is how I'm parsing it because of the... poor wording:
- You can use this ability multiple times, therefore renewing the shaken condition each time (not stacking it). All to try and "improve" one roll to a -1 modifier, but that's unwise. You're making every attack roll, saving throw, skill check, and ability check at a -2. Meanwhile that feat is only helping you reduce one roll's penalty to a -1.
- You apply the +1 bonus to your roll immediately (immediate action).
- Your shaken penalties are applied for any further results rolled after this particular roll is resolved.
By the description of the feat's goal you are trying to prove your worthiness in order to get your abilities back. So in fighting "challenging" foes you should be concerned about not proving your worthiness.
Fear of failing, or of further betraying your code of honor, are legitimate reasons to give your character anxiety when paired up against significant opposition (or insignificant, too, an unworthy challenge can sometimes be seen as having no honor).
Goal: Regain any lost class features and complete a quest to prove your worth. In the course of completing this quest, you must decisively defeat a challenging foe. The quest should relate in some fashion to the transgression that removed your class abilities or led to you being disgraced.