This is a common problem in instinctive aiming. It is easy at fairly short ranges, say 15 to 20 yards, for most archers. As you extend the range you can hold it together for a while but at some distance it seems to fall apart.
I think a different skill is needed for longer shots.
So what I teach is to move back even farther to your point-on distance. That is where if you put the arrow point in your vision right on target center, you will make a good hit.
This is a great distance to practice your form on, as aiming is simple. For many of us this is from 35 to 45 yards, give or take, depending upon your tackle and where you anchor on your face.
Practice there and when you get the hang of it, start shortening your range. I find working down to the distance that gives you trouble may work better than working down to it.
For me the tought distance was 30 yards. So for a year I did my warm-ups at 30, and at the end of the session I took some final shots at 30. I finally got the hang of it and now am better at 30 than most guys. - lbg