My girlfriend caught me cheating, how do I get her back?

Anonymous
I invited my ex over for drinks and while drunk, we kissed. She uploaded the photo of us kissing to Facebook and my girlfriend saw it. The next day, I visited my gf's house and she slapped me hard. She also dumped me for kissing my ex.

Did I deserve to be slapped?
And how do I get her back?
Updates:
+1 y
I was in the restroom while she uploaded the photo.
My girlfriend caught me cheating, how do I get her back?
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