I've never used a helmet mounted mirror, but those or the glasses-mounted mirrors are probably your best shot. These give a wider field of rear view, and you can adjust where you look just by moving your head.

I'd say to give the glasses-mounted mirror another try. In my own case, and others I've talked to, it takes a while to get the adjustment dialed in. Typically, people set them up using their ear or shoulder as reference points, without swinging it so far up or out as to make it uncomfortable to look. And not so far in as to interfere significantly with the forward field of view.