Is there any way you could go to the gym before work? sometimes its the only way i can fit a workout into to day, and i think it supposed to be the best time for fat burning exercise, also, its out of the way for the day and leaves you feeling good. I know its not always easy, especially in the cold dark mornings.
If this isnt going to work, keep with what your currently doing, the more exercise you do, the more energy you will have, and the longer you keep up the exercise you will get into a routine.
I would try eating a banana chopped into a yoghurt or a couple of mini babybels or even a light ham/tuna salad, all recommended with a strong tea or coffee. and plan a 'treat' for when you get home - even if its just a lovely hot bath or a new face cream/new clothes at the weekend if you manage to go so many times in the week. motivation for me is all about playing little games and dangling a 'carrot'!
any way, seem to have written an essay, sorry! just can really empathise with how you feel.
good luck. if you can motivate yourself in the bleak month of jan you can do anything!!!!