So for the longest time, I think I misunderstood "Focus on the front sight". This article opened my eyes, so to speak:
http://pistol-training.com/articles/vision

I'm not sure if this is related to what's going on with your vision, but it's worth a read anyway. It took some practice with it, but it's made a huge improvement in my performance. I think it's also easier to hold focus with your eyes converged at distance than it is with them converged at the front sight.

All also echo the suggestion to find a good optometrist.