An Awkward Situation

I'll try to keep this short. So, I(m-18) asked this girl(f-19) to prom and she said yes, but today, about a week later, she posts on facebook that she's in a relationship with this other guy. Reddit, should I tell her never mind and find another date, or should I just go with her and try to act like its not painfully awkward? I should add that I really like her, but I don't know what to do. HELP!