I think the best thing to do is to make a conscious choice to NOT give them any of your energy. They are not worth it, hard as it may be sometimes to let it go - I try to think to myself 'well it says much more about them than it does about me'. I get days like that too, and like LOTM says if I am struggling financially (ie paid out on a room) then I will also ditch the appointment system so I'll at least then get a booking to cover costs.
I second the above advice about having something nice lined up to do after work, even just something small you enjoy x