You've already told your spouse, your friends and anyone who will listen about your bad experience with Company X, but putting it into an actual complaint letter seems much tougher. Use voice dictation tools to extract the call to action from your story.
This scenario came up today at my coworking space. I had told two poor souls about a bad experience I had with in-flight Wi-Fi. I had a draft letter in Gmail written up, with all kinds of overly wordy points to make and a strangely robotic tone. A coworker, Nate, suggested something else: activate Siri and say the gist of what you need to complain about.
You want to be concise, sure, and you want to back up your complaints with key experience details and data, which isn't easy to write through dictation. But if it's less intimidating to speak out exactly why you're displeased and what went wrong, it's worth a shot.