Name the relationship as you experienced it
Acknowledge that the connection mattered and state that you are ending the romantic or sexual part. Do not use the lack of a label to pretend no explanation is needed.
Choose clarity over a list of defects
Give the main truthful reason at a useful level, such as wanting different commitments or no longer wanting to continue. Avoid a debate-sized inventory of their flaws.
Close the behaviors that kept it ambiguous
Return belongings, settle costs, change access, and stop late-night or intimate contact. Friendship can be considered later, not imposed as immediate consolation.