Getting through that frame of mind is really difficult. You could try keeping a
workout journal or putting up post its to remind you of the buzz you get after exercise or the achievements you've made. Likewise you can make plans to exercise with a friend at least once a week, even just a walk. The socialising will boost your mood as well as the exercise and it's a good way to prevent any unmotivated changes in plans because of the expectation to meet up.