I'm actually doing the same thing! I was very surprised when I realised why "lips together, teeth apart" does not work for me, my face is still supertense and sore - it's because even though I try to keep my teeth apart, I still tense my facial muscles.
It's like a vicious circle - feel the tension/pain, notice the clenched muscles, put a conscious effort to release it, and after a minute it's back, and here we go again.
Why do people keep doing this to themselves?