The image of "a huge mountain 'tween my heart and tongue" suggests that Portia does not want to say what she is feeling.
When Lucius asks Portia why she sends him to the Senate, she hesitates to answer him; her mind is put elsewhere. Portia wants to be strong enough not to reveal what she knows about Brutus' plans, since he considers Caesar has to be killed for power abuse. She cannot keep such secret anymore, and begs "constancy" to help her maintain composure.
