If you were doing every physical aspect correctly, you wouldn't have any mental issues. Mental issues develop from incorrect physical execution. That is not to make light of the mental aspect of the sport. I just got done mentally demolishing myself on a hunt in which I just could not make myself get the bow back. The mental aspect is a large part, however...
How do you correct a mental problem? Focus? Concentrate? Focus on what? Concentrate on what? Go ahead and double up on your Prozac... It won't help. You have to identify where you are breaking down physically, even if some elusive mental problem is causing the physical breakdown.
Fact is, the more you worry about a problem, the worse it becomes. And if you don't know what that problem is, well... That creates stress and the body's natural physical reaction to stress is to turn in to itself which is exactly NOT what you want if back tension is your goal.
Keep shooting away and in no time at all, your arms will be taking up the slack. You won't realize the change because you'll still be shooting good enough. Then pretty soon, you'll start releasing without actually thinking about it. It won't matter though because you still hit where you want "most of the time". Next your draw starts getting shorter. Maybe you creep, pluck, collapse, whatever... That's when you start worrying... and that's when that big angry monster we all call Target Panic jumps on your back and takes a bite out of your... er... backside.
At least that's how it went for me. Now I have to break down every shot and physically rebuild my muscle memory correctly. I suggest you take each shot for what it is, not where the arrow hit. Don't fix a shooting problem with more shooting. Fix it with smart shooting. Identify where and why you are losing back tension (which ultimately IS the problem) on each shot you do not execute correctly. Find out the why and fix it now. Focusing on concentrating or concentrating on focusing... or whatever it is you're suppose to do when you have no idea why something is happening is about as likely to work as a welfare baby.
And, as always... I could be completely wrong on this... So take it for what it's worth!