The image of "a huge mountain ‘tween my heart and tongue” suggests that Portia wishes for a barrier to prevent her from expressing her feelings and secrets. She is grappling with her emotions and the weight of the knowledge she possesses, and she desires strength to keep her thoughts and feelings contained. Therefore, the most fitting interpretation is:
Portia wishes for help to keep from saying what she knows and feels.
This reflects her internal struggle between her feelings and the expectations placed upon her, emphasizing her desire for control over her emotions and the situation.