If you´re sob-prone, write it out

Never email or snail-mail a break up letter, but if you know you´ll break down and be unable to talk, one option is to write out why you felt you had to break up, and bring the letter along. That way if you become unable to speak, you can still hand your partner the letter and sit while he or she reads it. Be there to talk about it (as best you can) and to answer questions, but this might help get you through the main part.