Don't be too hard on yourself. I am sure a lot of it, if not all of it, has to do with the shortening daylight hours, the dark encroaching upon your mood and the tendency towards hibernation some of us feel at this time of year. I am like you, a night time exerciser and I am feeling it getting very difficult to motivate myself and avoid giving in to the "well I'm yawning, so I must be exhausted, I'll just curl up on the couch with a packet of cookies and the TV remote."
Also, you are either a morning person or you are not. Multiple studies have mapped attention spans and energy levels and have detected marked differences in morning larks and night owls. A night owl will never have the energy the morning lark has if she tries to get up at 5.30 am to get her workout in. In fact, her chances of injury would go shooting up because she might try to push herself but lack the wakefulness and responses needed to avoid a fall/injury. On the other hand, morning larks wake up with good energy levels that start to diminish throughout the day whereas night owls show energy levels that gradually increase as the day goes on.
So, you are a night time person naturally, but at the moment, you are being affected by darkness and it is tricking your body and mind into believing that it is later than it is and that you are more tired, perhaps, than you really are. The only real answer is increased discipline to get you past this hump. Stand up, put your workout gear on and make a deal with yourself: start a workout, pick one that lifts your mood and is pure fun, not one whose dread factor shoots through the roof, and if you are not humming along nicely after 20 mins, then cool down, stretch and take a nice bath. At least, you tried and did not give into the dreaded lure of the couch!
Also, remind yourself that fitness is not a competition, a lot of us are struggling with the exact same feelings and tendencies right now and it's OK to do 3 workouts per week in the winter, not 6 or 7, if you just can't motivate yourself to do any more. 3 will still keep you in excellent shape if you gun it once you get into it.
Clare