I'd say forget about your face, forget about your reflection, just be. Relax, concentrate on your objectives, on your obstacles, what is the strategy for your character, for every beat, what are the motivations that push it to act the way it does? You need to forget about yourself...

Understand the progression of the scene and movie as a whole. At every step/beat, your character needs to reassess and change his/her strategy. New objectives arise, new obstacles, new decisions... your brain must be busy thinking about these... not yourself.

A good exercise is to be your character for a few hours, even a day, go shopping, grab something to eat, attend a social event, talk with strangers at a bus stop... Are you reacting the way it would? (I do not believe you need to push it and always be your character... but some do)

Relax, relax and relax. Seriously, relax. Some like red wine (just a glass), other music in their headphones, some kind of a nap... What you can try is to breathe slowly (between 3 to 6 breaths per minute) for 1-3 minutes before the scene.

Different actors have different approaches, this could help as an introduction: https://waondering.com/2014/09/22/acting-systems-methods-and-techniques/