Unfortunately, you are in the midst of a learning, or rather, a re-training, experience. You have gotten used to sleeping 'naturally' in various sleeping positions with varying head attitudes and positions relative to whichever pillow you are currently using.
I had the same experience, feeling I had little choice but to get used to falling asleep on my back. That was what I was faced with, and eventually adapted to, for the first seven months of my own therapy. Then, suddenly, I found I could fall asleep as I used to, on my side. I wouldn't get the deflection of the cushion with its formerly attendant lifting and breaking of the seal due to deformation of the cushion or slipping of the headgear straps as my head settled into the pillow under friction.
Essentially, you will have to learn what you can and what you cannot do...for now anyway...in the way of sleeping position as you fall asleep and later on as you turn naturally without realizing it. I think, and hope, that you will eventually learn how to adapt, even if it means, as it does for me, briefly waking to adjust a slipping hose, or to adjust the pillow, or to move my head differently to prevent leakage, or even turning over to reorient my body entirely.
As stated above, the cushion will not stand for tight headgear any more than it will for over-loose strapping. There's a sweet spot, with minor variation, at which you will be able to sleep almost naturally with impunity. You're just not there yet.