The unsent letter may tell the truth about one hour rather than the whole relationship.

Something written in a bad hour is accurate about that hour and unreliable about everything else. The anger or the longing behind it was real and also partial, catching one narrow slice of a long connection at its sharpest point. Not sending it is often the correct reading of that limit.

Write the letter when you need to, then leave a night between writing and sending. Read it again in the morning and ask whether it describes the person or the evening. Keep whatever part is still true and let the rest stay where it was useful.