I am extremely scared that my best friend lies to me about being my best friend and secretly has other best friends.

This guy I think is trying to steal my best friend from me. He needs to get his own best friend. Not steal mine. I'm extremely upset.
Edit: I know maybe it doesn't suit my age to post something like this but it's a void I've lived with since I was a child. The friendlessness..... People just... Leaving. And a time came that it started to hurt.... And now it's become something I can't let go of. I can't let happen to myself. It's not fair that other people around me get to bond much better with one another... To the point my friends would choose new friends over me? How is this possible? What is wrong with me? Why am I not good enough?