I agree that it's mentally, physicall and emotionally very taxing to keep doint the hard sessions. Just 10 days ago, I pulled out of a session because my head just wasn't right. I was thinking negative thougts, so I did some sprints and the next day a long hard run, which I really enjoyed.
I think I was doing the right thing by not attempting to complete the session with the wrong mindset, and a few days later, with a more positive mindset, I completed the session. I think that if you try to do a session when your head isn't right, you are setting yourself up for future failure, because that negativity is bound to return and soon you will begin to associate negative feelings with running hard, when the opposite mindset is required.