How can I stop thinking about my narcissistic ex?

I don't want him back, ik very well it is a blessing not being with him. But i get this feeling sometimes like unfinished stuff, the truth is i can't believe i trusted a guy like that n how can a person do that to another person, this is how i feel n its bugging me sometimes whole day. How can i pas through this?
How can I stop thinking about my narcissistic ex?
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