Once the speaker was young — now matured — duty to turn his attention to harsh realities — casualties caused by war, slavery — epidemic diseases — curses of countless centuries — exploitation of poor, invalids, slaves.
Detailed Answer :
Once the speaker was young, he loved his beloved deeply. Now he is matured. He comes to realize that there are other truths to be attended. He honestly tells her that his love would be same but he can't spend the time for her because his duty is to turn his attention to harsh realities of life casualties caused by war, slavery, epidemic diseases, curses of countless centuries, exploitation of poor, invalid and slaves.
