I want to get back with my ex I don't know what to tell her

Tell her you want her back. That's all you can tell her.

Good luck.

Is that all the info you have??

(My little play)

Riiiiingg...

"Hi, Are you vacant? I want to go out see a movie. What would you like?"

If she agreed...

"Would you like some ice cream?"

If she said yes...

"You're glowing so I wonder if someone has already taken my place."

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Seriously buddy, We don't know your GF so we need more information.