My first boyfriend’s engagement announcement recently popped up on my Instagram feed. I felt a weird mix of emotions and wrote him a two-page letter in my notes app. I soon realised that sending him this after eight years of no contact would be insane, but it got me thinking about all the things left unsaid in past relationships. So I invited others to share their bittersweet lessons, sassy one-liners and declarations of love from their breakup letters destined for the bin.