One of the emotionally hardest things for anyone to do, is to write a goodbye or farewell letter. This is because this letter is usually written at the end of a long term that you may have served at a specific place. It also means that you will be moving away from the place that you resided in for a set amount of time, while moving away from the people who you spent a decent amount of time with prior to your departure.
When writing a farewell letter people tend to either forget some key elements, or tend to fail to convert what they are thinking onto paper.
The opening paragraph should talk about why you are leaving, and highlight all the good times that you shared with the people who you are saying goodbye to. The following paragraph/paragraphs should wish everyone good luck, and give them your contact information in case they want to get in touch with you later on.