Drill to keep left arm straight..

Alex, i have had this and was obsessed with keeping it straight

In fact i found it was my right arm restricting my backswing therefore my left would bend and i would overswing, the plane was very flat and behind me, horrible

Look at your swing plane and right arm / left arm triangle at the top and as bob suggested 3/4 swings

When you have mastered that then try downswings without using your arm and only the lower body !