How can I walk away from this one-sided friendship without seeming immature about it? ?

I've been friends with this guy for a year now. He's very selfish and self-centred. I've always been there for him and supported him, but he's proven he'll never be there for me. He's a user and manipulative. I think he may be a narcissist. He always gives me backhanded compliments. It's taken a year to see through his fake persona.

He has a negative effect on my mental health. I need to end the friendship, but not sure how to go about it without being immature about it.

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How can I walk away from this one-sided friendship without seeming immature about it? ?
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