How do I nicely tell my already-shunned best friend that there's no room for her in my New Year's Eve plans?

Her other “best friend” has this bad habit of not inviting her out when she and her boyfriend make plans with other people. Not mutual friends, not couples… just people. And she leaves my friend out. My friend is hurt, so last night, she sort of invited herself to whatever I’m doing New Year’s Eve. Which isn’t rude, since it’s never a problem when either of us invites ourselves to anything… we’re best friends… that’s what we do. Except my boyfriend will be here New Year’s Eve, and frankly, there’s only room in the bed for two :x

She’s already been pushed aside by one of the two “besties” for a boyfriend. How can I do that to her too? I only get J for 11 days, and I’m sorry, I’m not willing to sacrifice any alone time. How do I tell her?