A letter gives the reader a pace the speaker cannot interrupt.

Speech happens at the speed of the person talking. A letter hands the pace over: you can stop at a difficult sentence, go back, put it down for an hour, finish it when you are able. Nothing pushes on while you are still absorbing the last part, which is why hard news often lands better in writing than in a conversation someone else is steering.

If there is something you have not managed to say out loud, consider writing it instead. Not to avoid the person, but to give them room to receive it in their own time. Then be available afterwards, when they are ready to talk.