Say the hard part early in the conversation, not with your hand on the doorknob.

Dropping something heavy at the exit gets you the relief of having said it while leaving the other person no time to answer. That is not confession, it is delivery, and it usually happens because the whole conversation was spent working up to it. Whatever they needed to ask has now become unaskable. Timing decides whether you had a conversation or made an announcement.

Decide the hard sentence before you walk in and say it in the first minute. Then stay for however long it takes. If you genuinely run out of time, say so and set when you will finish it.