Have you ever thought about joining a fitness class? I feel like it makes you feel accountable especially if you have a friend that goes with you. If you carpool, then it is even harder to skip out on the exercise!
Also, I sometimes get the winter blues and find hard to be motivated. I love to go running outdoors but due to cold temps & sometimes icy pavement, I just don't want to go out there.
Another idea, try something new. Something new can be exciting and invigorating. Maybe it is not just the exercise, try a new healthy food. If you are eating right, maybe it will help you feel better as well.