The art of losing isn't hard to master;
So many things seem filled with the intent
To be lost that their loss is no disaster.
Lose something every day. Accept the fluster
The opening is a contradiction, an  oxymoron , in which the speaker claims that losing is an ‘art’, that can be ‘mastered’. It is clearly an ironic understatement, as loss, by definition, is a painful state of grieving for something one no longer has. The poet seems to be preparing the reader for a complicated line of reasoning.— that something we know is hard and painful is precisely what she claims is easy.
One Art