Hello
Maybe just relax, you might be putting too much pressure on yourself every time you have a +12ft putt. If your putts are ending 3-5 inches away from the cup you don't have to worry, it's just unlucky. Apart from hitting the ball just a little bit harder or softer, I don't think you should change anything... one day all those putts will start to drop. Maybe have a few practise games on 'Tounament Greens' and see how you go. These are a bit faster than the 'Very Fast Greens' . The Green Speeds are .... Slow, Standard, Fast, Very Fast, Tournament and Championship.
Have you read the 'Putting Distance Control' post in this section, that helped me alot.
Cheers
Roger