How to forget cheating?

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My GF cheated on me last year , i was seperated from my wife for 3 years while i was meeting my GF , i caught her through her FB inbox and when confronted her she said she did it because i wasnt divorced yet (only seperated) and didnt feel i was truly commited to her , we are back together and i forgave her but honestly i cant stop thinking of it , i am constantly giving her a hard time and doubting her everytime she moves her lips , how can i truly forgive and forget....