quiet+pain

Regret? Sadness? Selfishness? This line could mean multiple things. She feels a "quiet pain" that cannot be understood by others. Considering this line comes right after she waits for someone who is not there, it could indicate that she has either been left by someone or is just sad that there is nobody in her life at the moment to fill the void in her heart. This void has constantly been filled by others in the past, as the beginning of the poem indicates.