My co worker invited me to her daughters 15th?

She invited me and I have to RSVP by today or at least that is what I told her. I want to go but then I don't because I'm not that close to her. Honestly I am shocked that she even invited me to go. I would still have to buy a dress and get a gift. Not sure why I am overthinking it maybe because I feel a little bad not taking my boyfriend, he said he is okay with it because I like to keep my relationship at work private because most people already know that we are dating plus I think it would be awkward, anyways my good friend said she can go with me so I'm thinking of going but I don't know why I'm overthinking it so much

My co worker invited me to her daughters 15th?
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