I rejected a guy, now he is talking shit about me to everyone. I don't know why he is doing that? He actnice infront ofme but backbiting me?

HELP ME!! I don't know WHAT TO DO?:/ PLEASE READ THIS. I met a guy who was acting nice , geniune, friendly sweet. He texted me first. We talked, we became friends. then bff's. Initially I liked him too coz of his nice nature , he likes me too but after sometime, I lost feelings for him. But he wasn't understanding this , and keep texting me and pushing me to date him and come in relationship with me. He was emotionally pressuring me to date him. I rejected his idea and said I can't date anyone rn let's stay friends not more than that. I already have severe mental issues from my past. He said -" okk I got it "and replied to me in a nice way. After some days, one of my friend (I mean my girl bff) texted me and said" that guy is talking shit about me to everyone and making false stories about me. She sent me ss. I was shocked coz that guy literally backbiting. He was saying shitty things about me to everyone that we (me and him) are in a relationship from 6 months and I cheated on him with another guy, I'm a lier , s**t and he is heartbroken blah blah blah, he was saying bad stuffs about me to everyone , even a random stranger. Literally spreading fake rumours , Wtfff I never dated anyone in my life 😭, so how can I be in a relationship. What relationship? Bruh we never dated each other so how can we be in a relationship? After seeing this , I was shocked and feeling sad. I confronted him, but he was acting nice and trying to be a victim and said - "we shouldn't hate each other , if I didn't date him , and didn't come in relationship with him, he will do the worse".
Wtfff He said those words to me and it was scary. My mental health is worse rn. I can't think properly. I thought he was really a nice guy after seeing his reality , I think I can't trust no one. He is pressuring me with different social media accounts rn and I don't know what to do?
I rejected a guy, now he is talking shit about me to everyone. I don't know why he is doing that? He actnice infront ofme but backbiting me?
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