Neruda "tonight i can write..."

I loved the poem tonight i can write by Pablo Neruda.  It was full of many emotions.  But mostly just how is relationship evolved from one thing into another.  He starts off saying tonight i can write the saddest lines and then repeats himself three more times.  Ins the poem he does this as a means to build up courage and to ensure himself that what he is doing is the right and true thing.  He talks about how he love a girl and how she loved him.  
Every stanza is in couplet form and he does not break the pattern (very rigid).  He is very sad as he reminisces about this lost love, about everything that he once had is now gone.  But eventually he emerges on the other side of the battle and becomes at peace with the last lines saying, "this be the last pain she make me suffer/and these the last verses that i write for her."  He finally comes to peace with his heart ache and is now controlling his life again, he not allowing someone else to dictate how he feels.