I am assuming you will be writing about English poetry. One approach to this essay could be to examine the themes of poetic narratives about lovers parting. How was life different before the twentieth century in ways that would make it necessary for lovers to part? Consider that telecommunications did not really exist, so the main way lovers could communicate at a distance was by letter. This placed more value on letters as forms of communication, but it would also be hard to feel close to someone without the ability to talk by phone.
There were many reasons lovers had to part prior to 1900. Sometimes families disapproved of a relationship, and young people were expected to obey their parents and to accept their fates, such as arranged marriages. Sometimes lovers had to be parted because of economic realities; for example, many young Irish men decided to emigrate to America during the famine years in order to make money to live and to support their families. Despite promising that they would return to their lovers, the harsh reality of travel and life abroad made this unlikely. There will be many examples in the poetry of this era that point to reasons why lovers had to part ways.