as far as jaw-relaxation goes, try putting your hands on either side of your face, and focus on breathing technique while you do vocalization.
You will have to completely focus on the process of singing, instead of the things your brain wants to do to make it sound the way you want. It may sound weird to you, but as long as you are not straining, or in pain then you are ok. As far as comfort on those high notes, that's only gonna come from practice, and proper practice.
So, in conclusion, practice for technique, not the sound you want, and practice OFTEN. (40 minutes a day, of good vocalization).
I hope that helps (I have the jaw thing too!)
and all that I could do because of you was talk of love...