How does commercial whole grain bread stay fresh for so long?
Industrial breads use 'dough conditioners' that soften the dough and make working with it easier. You can buy such mixes online, here for instance, and I've seen them for sale in 
natural food stores and the like. Other things that might work are adding a starch or a fat, or heat treating some of your flour in the microwave (a minute for a cup, don't do this to all the flour, it destroys some of the gluten). Guar gum or xanthan gum will help to keep things moist as well.

I've never used these techniques, so some experimentation might be necessary. Store bread in plastic bags as soon as it cools, and don't slice right away.