Isn't there a sport you can enjoy playing? Surely if you get yourself playing something then it kinda forces you to keep going back even when you feel like a night by the fire eating crisps?
Maybe the problem you have is that your keeping fit by doing stuff you just dont like doing? A lot of people equate "getting fit" to going out running. I obviously have no idea what you do but a lot of people just dont enjoy running to get fit. There are other ways though. Walking, swimming, tennis, badminton, football, whatever. Finding something you actually enjoy doing will maybe help?
I play squash but while it does keep me fit I also train for playing. So the weight stuff I do and the leg work I do is specifically for that. I probably wouldnt be motivated to exercise otherwise. But I actually enjoy it and I feel like I am cheating if I miss it out for a few days.