Letter is a fantastic idea and one I would probably do myself since I have trouble vocalizing my thoughts, especially when I might get emotional.

Some people prefer to talk, so if that's you then I would definitely just ask them if you can have a serious discussion with them and then let it out. Whenever I'm going to have a serious talk with someone I make notes about the talking points beforehand. It helps me to stay on target and make sure I've gotten everything said that I need to say.