Disclaimer: I don't own Doctor Who. I'm pretty sure it owns me.
"Do you honestly think she'll go for this? You think she'll just turn her back on you, after all she's done to get back to you, and ride off into the sunset with some strange copy?" The Metacrisis spoke bitterly, knowing the Doctor's decision was already made.
"I can't give her the choice. Someone has to take care of you, and I know how you feel about her – you will show her all the love I have, the love I can never express, " the Doctor replied gently. "There is no happy ending for her with me. With you, she can have forever, the rest of her life. You can grow old with her, be her partner in a way I never could."
"You're assuming I can convince her that I am you, or near enough!" shouted the Metacrisis. "You're assuming that after you abandon her, again, I can keep her from breaking into a million pieces, from struggling with that Dimension Cannon til she gets herself killed trying to get away from me and back to you again!"
"Just love her. Be patient, be strong, and be who you are. She will love you in the end, and I know the two of you can have a life together."
"You're an idiot." Now the Metacrisis was in tears. "She loves you, not me. She will never forget you, and you are dooming me to live in the shadow of her memory of you for the rest of my mortal life."
"I will do the best I can to make this easy for the both of you." The Doctor had tears in his eyes too. "This isn't easy, but it is for the best. I can't give her what she needs, and you can't be left on your own."
The Doctor put his hands on the other man's shoulders. "Have a wonderful life with her. I wish it could be me, you know I do. But this is her best chance at happiness. You have to make her believe that."
Looking in the Doctor's eyes, the Metacrisis saw how sincerely he meant these words. He swore that he would do his best to make Rose Tyler happy for the rest of his existence.