How is memory shown?

The theme of memory in Tears, Idle Tears by Tennyson is shown as both beautiful and painful. 
The speaker contemplates his life and thinks about past days, the "days that are no more," which were beautiful and full of passion but now have passed. Even the happiest moments are temporary and must end, leaving only memories behind. For this reason, they are truly beautiful but also tinged with sadness and melancholy, as the speaker knows it will be impossible to relive these experiences. These past times and memories only live on in our minds as time goes by, causing "tears, idle tears" when they are remembered. 
This poem focuses on the theme of the passing of time and the consequent realization that everything must end in death, especially our own lives. It is this poignant realization however that pushes us to create beautiful things and truly appreciate life, as we know that we will not get the chance to relive certain experiences.