State the limit without diagnosing yourself
Say that you cannot offer the relationship they want now. You may give brief context, but you do not need a dramatic story or a list of their qualities to justify the decision.
Remove the future promise
Do not ask them to wait, behave like a partner, or remain emotionally exclusive. If you want casual contact or friendship, name it honestly and let them decline.
Make your behavior match the message
Reduce romantic routines, physical intimacy, and daily dependence if those actions contradict the boundary. Clarity is a pattern, not one carefully worded text.