I'm upset with my coworker getting a company car
Why can't you just pretend to be happy for her?
I should but I can't fake excitement. I don't want to act like her.
I think in situations like this it's expected for you to just be polite and congratulate them, regardless of how you really feel.
It's hard, because I am an antisocial person.
Some personality traits can't be changed. If you let people pressure you into being social, it may make every one uncomfortable to realize you are being forced into the situation.