Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to apologize for dominating every conversation, use this factual base: identify the moment, the impact on trust or inclusion, and what your friend should not have had to carry.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Offer a repair that matches the harm and show how your behavior will be different. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not use shared history, mutual friends, or public gestures to pressure a private response. If contact is unwelcome, stop and avoid using mutual friends as messengers. You do not need the other person to agree with a boundary for you to follow it consistently.