It certainly sounds like you've tried a lot. One approach could be to (and I'm not being facetious) cultivate a mindfulness-in-boredom practice. Seriously. I'm not saying you will initially get onto the treadmill thinking "Boy, I can't wait to be bored." However, over time you might start to crave that time in your head. Alternatively (if that sounds absolutely unappealing), you might try another type of exercise. Running definitely isn't the be all, end all. I wish you luck!