I have always had a bad habit of putting outside in, cutting across the ball, with my body set up facing left of the hole. I seem to eliminate this if I take the putter back very short with a longer follow through, which is especially effective on the short putts. It does not however encourage stroking the ball which you recommend. Grateful for your comment please.
I am sorry to hear that you are having problems with your swing path when putting. Unfortunately the fixes you have tried may work in the short term but will only lead to further problems down the road.
The putting stroke is really very simple if we follow some basic fundamentals and persuade the mind to give up instructional control.
These fundamentals are good feet and body alignment; eyes directly over the ball; and rocking your shoulders which are connected to the arms, wrists, hands, and the putter, as if it was all one fixed pendulum-like structure.
To correct your problem make sure that you are correctly aligned and then swing the structure described above by using the shoulders as the engine to initiate the stroke. Do not take the putter back with your hands and/or arms.
Nic, I hope that this explanation of the pendulum-like “structure” and letting the natural forces, not your controlling mind, dictate the rhythm, makes sense, as it is a natural, not a contrived or controlled stroke.
If you apply this and practice it in your living room, office or elsewhere your swing path will fall in place naturally. Most importantly if you swing the putter head over the straight edge of a carpet you will see an arc like stroke path.
Nic, I think you will find my latest innovation, The Frog Stick, very helpful. By snapping it onto the shaft of your putter you can see the natural arc-like stroke when you follow the fundamentals above. This will allow you to improve your putting quickly and develop a good and natural swing path, along with excellent rhythm.
To understand more about how The Frog Stick works be sure to watch the video.
Good luck and have more fun on the green.