a lone wolf
A lone wolf is someone who doesn't want to be part of any group, even if it means being completely ignored by everyone.
He went to the mall by himself. No friends. No snacks. Just a sad, angry, middle-aged man with a broken coffee cup.
She joined the gym by herself. No one talks to her. She's just there to punch a bag and scream at the ceiling.
He tried to join the club. They all said no. Now he's just a sad, bitter man in a hoodie.