Writing the letter will be a good way for you to take things of your chest. The other side of it is, that they may not respond in a way that you hope they would. They may not acknowledge your feelings or see a valid concern and that would only make you more disappointed. Never the less, it is a reasonable to write to them and point out how their behavior affects you (not placing blame) but still letting them know what you expect as a parent from them. Just keep realistic expectations that even if they do not respond in an agreeable way, you still had shared something that is important to you. You won't be able to change them and sometimes no matter what, people continue to act their usual way. Don't blame yourself for this either. They are adults and had decided how to behave.