"It ended suddenly as is common for me - a defensive response"

Who ended the relationship? You - or her?

"I want to find some closure."

You may never find closure, unfortunately....

"Have you experience of writing a post-break-up letter?"

Generally speaking once my relationships have ended, they have well and truly ended, particularly when ended by the other party.

" I want to leave the door open to a possible reconciliation."

Just don't get hurt!