How do i cope with losing a best friend?

It's not what you think. We've been friends I want to say 10+ years he's been a phenomenal friend nothing bad happened between us. It's just he's finally found love and as expected we don't spend much time together.

This doesn't offend me at all. I understand and I don't want to impede in the growth of their relationship. I want my friend to be happy and find happiness even without me in it.

As for me, it's just hard to cope because loneliness is a theme of my life no one stays for long. Every new friend I make fizzles out. He's been the realest most constant friend so this hurts.

How do I move on like an adult and be okay? I'm 33 and feel like I'll be alone and friendless and without a companion ( romamtic) forever. I don't have much hope of my life ever changing...

I guess I'm asking how to cope with the loneliness?