Should I write an apology letter after all these years?

Feeling guilty is a state of mind and as such, you can choose not to feel guilty even when you are obviously guilty. In your case, i suppose you are haunted by guilt propelled by awareness-of-etiquette. It wont do anything bad if you take a couple of minutes to write a letter of apology to them (just for the sake of writing one) and free yourself from the leash as it is usually good to maintain decorum against future meetings. Hope this helps.