So i feel bad for feeling this way

But my best friend since the 7th grade (we are now 21) she recently got into a relationship and I’m happy for her. We used to talk non stop everyday and now we barely talk and when we do talk it’s only ever about him. So i guess what I’m saying is I’m lonely and i really don’t know how to handle this feeling. I don’t have a boyfriend nor do i really have any other friends. So the loneliness is making me sad and easily frustrated and then i go into this thing where i start to over think and all this other stuff and just brings my mood down more. I don’t know what to do.