Choose a private place to write
Use a notebook or a document away from the live conversation. Give the page a clear purpose: to hear what you are feeling. You can write honestly without deciding that the other person must receive every sentence. There is no requirement to share this draft.
Find the sentence underneath
After a pause, read the draft and look for the sentence that describes what mattered. It may be 'I wanted a clear answer' or 'I need more notice when plans change.' This is often easier to work with than a long list of accusations, explanations, and hoped-for replies.
Decide whether anything needs sending
A practical question may benefit from a short message. A private realisation may not need an audience. Ask what the message is meant to accomplish, whether the request is clear, and whether you can accept a range of answers. Don't send a message as a test designed to provoke guilt, jealousy, or pursuit.
Keep your choice
You may send a revised version, have a conversation, keep the page, or delete it. None of these choices proves how deeply you felt something. Choose what serves your current needs and respects the other person's autonomy. If contact could put you at risk, get appropriate support before reaching out.