It depends on what ¡°Goodbye¡± really means in this context. Also about ¡°how¡±? You could be asking ¡°how do I have the strength to do this¡±? Also could be, ¡°how do I do this well?¡±, or even ¡°how do I do this so they will come back?¡± or even ¡°they are about to die—what do I say?¡±

Since I can imagine all sorts of scenarios, and I don't know who you and this other person are, I will sum up generic advice. Be strong, be courteous, and honest. Basically be honest no matter what, but do not make demands or ask anything of them. Just let them know how you feel about them, and say that you will do/accept this because you care. Find your own words to say these things.

But remember that trying to manipulate or control them will only backfire badly. Deal with your feelings on the subject in private, but don't be a robot in this person's presence. It's OK to cry, but don't make your feelings their problem.

Remember that other people care whether you live or die. Yes, even complete strangers you don't know exist right now. Live for them and for your obligations until your heart heals. This too will pass, and making big decisions when you are this hurt leads to dark and irrevocable decisions. I've been there, and I don't regret deciding to live anyway.