I wouldn't say not training hard enough, perhaps you just need more stress. This is just my opinion, but tunnel vision is induced by stress. So the more you train under more stress, the more you will adapt. That is why the search for more realistic training is ever ongoing.
Knowing what causes it and that it is happening is half the battle.
Scanning, continuous movement, etc. are all good practices and help.
Try doing SIMs with more than one opponent. If you lose, you have to bend over and take a free one in the ass. Very good for inducing stress.