Sentence:  He thought how he would tell her . . . that his love for her had fallen off through his own unworthiness, and had returned to one who was in all respects less perfect than she, but who in old days, as she well knew, had been his <WSD>first love</WSD>. 
Meaning of the word marked with <WSD> tags:  The first person to be the object of one's romantic affection.