I'm secretly upset with my best friend and new roommate for inviting my ex over without asking and now I feel like an absolute loser
Why do you think she did that?
I think she doing it to show off. She does a lot of this stuff to "impress" people so she could look good
They should have at least talked to you first and let you know. That's not something you do to a friend of yours.
I am going to get back at them for this. My plan is to make them look like the stupid losers. That will show them.
You did not like it when they made you seem like a loser so you shouldn't do that to others. It is better to be considerate of others even if they are not to you. Explain what you feel to them better.