I'm going to give my ex an honest tenant recommendation.
Why are you going to give them that?
I really miss him, I hope we can get back together one day.
It is good to be honest with other people in all dealings. Even even you miss someone who ended your relationship it is noble that you give them a good word.
I'm going to hurt him by writing a bad reference letter and thinking of all the ways I can make his new girlfriend run out.
You shouldn't do that, stick to your original plan and just be honest. It's the best way to live life.