Today I had an terrible phd interview, it was my first interview and I thought I could ace it but it turned out to be a mess. During the preparation phase, I even wrote down 10 questions that I might be asked; I was expecting the interviewers to ask me something related to my future research or my motivation for being there. Sadly, they didn't ask me that; however, my questions, for the most part, were about my master thesis, and I thought that I was having another master oral defence. It was a real surprise that they even asked me how my master thesis project can be applied to artificial intelligence, but my major is cognitive linguistics not computational linguistics, and seriously I didn know how to answer it.
This bad interview experience has already spoiled my mood to prepare for the next phd interview, I know I should brush it off, but the image of that interview moment keeps popping into my head, what should I do now?

It is your first interview, so don't worry about it. Every interview is different and you shouldn't expect yourself to be perfect on your first attempt. Take it as a learning experience and build from it. You can't predict every interview question but you can prepare the best as you can and learn for next time.

If you are having trouble motivating yourself focus on the end goal not the obstacle. You want the PhD so the interview is just something you have to overcome.

Goodluck!

Thank you rewt, your kind words make me feel better right now. =)