Congrats on your promotion interview @luz! That’s really exciting 
It sounds like your nervousness is the main obstacle in your way, so adding to the great suggestions others have offered I’d recommend wearing something to help give you a boost.
A crystal bracelet/necklace (Christina and Fabian shared some great choices- I also recommend Lepidolite for easing anxiety), or a piece of jewelry cleansed and enchanted with good energy 
are good choices.
You could also use sigils: either the Sun Sigil to Pass a Test (tuck the drawing into a pocket and bring it to the interview with you) or by drawing the Self Confidence Sigil in the air just before you go into the meeting.
And a few tips from meditation that have helped me (someone living with anxiety)- don’t underestimate the power of deep, focused breathing and holding good posture before and during the interview. A bit of mindfulness with your breath control and how your hold your body can really help to give your self-confidence a boost! 
Good luck, Luz! 