When my focus on the front sight gets blurry, which it can do after a few mins due to age, I fall back to using the Force, and groups go to heck. I wish I could be accurate without focusing on the front sight, but I can't, short of optics.
Shoot a couple mags and take a few minutes break. Pace. Over time you will be able to shoot longer strings. Breath lots and deeply. Lack of oxygen will fatigue your eyes quickly. And your muscles obviously. But eyes fatigue quick. I found squinting my non dominant eye for a couple seconds then relaxing helps my dominant eye regain focus as well. Try to look at your shots after your mag is empty. Don’t worry about placement until then. This will keep your eyes from hunting for focus so much. I hope some of this helps.