How to cope with the loneliness after a party?

Focus on what was fun about the party you had and stop comparing it to the "lonely" part of the situation afterwards. Like some have already said, worrying about the end will only decrease your ability to have fun while you dread about the end of the party.

Focus on having fun preemptively when you are at the party, and then when it's over, reflect on what you enjoyed, what was fun about it, or if there was a particular song/track/mix you liked about the music the host played, see if you can explore and collect that while you await for the next party invitation to come up. Just try to focus on being productive from feeding off of "good" memories and don't drone so much about how you feel about not being with your group of friends afterwards. They are your friends after all, aren't they?

Jam it back in, in the dark.