I had exactly the same problem last year and it occurred after I had been shooting quite happily for 6 months. I tried all sorts of techniques including standing 2 yards from the boss and shooting with eyes closed and 'feeling' the shot. This worked fine until I opened my eyes to shoot and it was back to early release again.
In the end it was suggested that I try a clicker and I've never looked back. It sorted my problem immediately. I don't know whether this would work for everyone, but it certainly worked for me.
