Interviewing can be hell,that's true. I tried to think of it like I was just having a conversation with someone who was really interested in knowing me.That helped me feel less like I was trying to sell myself. I also tried making it a little bit like a conversation and tried to find some common interest or common ground to talk about. My field is socialwork,so there was always an interviewer wanting to talk about social policy or philosophy.
I'm sure you've thought of these things,but sometimes it's good to remind yourself of them before you go on the interview.(If I didn't think of it ahead of time it would be more difficult for me,because I'd be agonizing over the outfit I was wearing. Which looked so much better before!)

You're doing this because you aren't happy at your job,and you feel things are going to get worse.
I'm sending you positive thoughts that you find something that makes you happy :)