I wish I could tell you what to do to feel better, but I do have the urge to say that what you are feeling strikes me as very, very normal. I know those feelings of ending a bad relationship, KNOWING it was overall bad for me and that it was the right thing to end it, but longing for the intensity and the good stuff. Of course you are feeling the absence of that intensity.

You're grieving, and your distractedness, lack of motivation, need to work through things that you can't really work through with your exT... these are all really normal and there's nothing wrong with you and no reason to be frustrated with yourself.