When you sing do you tense your lips? Are you exaggerating their movement or that of your cheeks or the opening of your mouth? Do you try to form your vowels with your lips?
Try this with a song you like to sing or perform.
Stand in front of a mirror and watch yourself as you sing. Really put yourself into the song and watch your face, especially your lips and mouth. If you exaggerate the movement of your face as part of achieving the notes, this tension will back up into your throat and you will find yourself pushing against this tension.
Remedy: Gently place the palms of your hands against the right and left cheeks of your face just to the sides of your mouth. Sing the song again and let your hands help you relax the movements of your lips, cheeks and mouth. Of course there will be some movement but we are working to have it be relaxed and natural, not excessive.
Does that feel better? Do you notice a difference in how you sound? It should feel easier to sing and it sound better.
Facial movements are a natural expression of your emotions. Let them happen but don’t use them to try to make the sounds of your voice or achieve the notes of your range.