I usually rock a barbarian so i just shout or glower for charisma.

But if you’re honestly struggling then i’ll recommend something i used to do to gain confidence. Pretend to be someone else. Like when Abed pretends to be Don Draper. If you act as someone suave then eventually it won’t be an act.

The one caveat is be careful who you’re emulating. Don Draper is smooth but he’s also a giant misogynist. Unless that’s the character you’re playing as, then go nuts.