Bud,In general you are looking pretty good. If you are feeling the "tired" where you say, you are getting into your back OK. Looking at your group, I can't help but think that your speed of release is "getting" you a little. It doesn't take very much string hand deviation at all to produce a horizontal spread like you show in the picture. To tighten that, you might TRY stopping at anchor just a little longer and assuring that the bones in your hand are pressing the bones in your face. Less than 1/8th inch of difference in your string hand position at release can easily cause that group spread.
Arne