Demotivated? How to deal with your fitness demons

It's 6pm: you've finished work for the day and are looking forward to hitting the gym. You've been working hard and the results are paying off. Yet, despite all this, the second you set foot outside, your motivation starts to wane. Sat at your desk- eyes glued to an Excel spreadsheet - the gym had seemed like a welcome break, but now, when the alternative is a warm house, the sofa and a slice of carrot cake, getting sweaty suddenly doesn't seem the best option. So, is there anything you can do to tackle your demotivating inner voice?