No worries! Another thing that helps me is the knowledge that I don't have to be 100% motivated all the time. Weight loss is a long journey, and weight management will take a lifetime. It's okay if your motivation wanes now and again. It's perfectly natural not to be fired up all the time. If you're truly in the doldrums and have no motivation whatsoever just accept that it's natural and that it will pass. Do what you can to remind yourself why losing weight is a great thing, but don't punish yourself over not being 100% driven 100% of the time.
Sometimes, though, it's a question of 'fake it till you make it'. You might not want to go for a walk after dinner, but if you make yourself do it then after a few days it starts becoming a routine. And after a month or so it becomes a habit. Then you'll find you look forward to your walk after dinner and you feel yuck if you don't go. And rather than feel miserable while you force yourself to do these things tell yourself that you really do love it. Plaster a smile on your face and do your best to feel it. If you don't feel like doing exercise, don't think that you have 0% motivation. Look on it as having 49% motivation. And the times you're really motivated? That's not 100% motivation - hell only olympic athletes have that - think of your most motivated times as being at 51%. Suddenly becoming motivated isn't such a daunting task. It's a matter of shifting your attitude only by 2 percentage points, and that's not much!
So I guess my advice would be:
1. Don't beat yourself up over a lack of motivation
2. Use visualisation as instant gratification to trick your mind into wanting to do something
3. Consider that it's only a matter of shifting your attitude from 49% to 51% to become motivated and excited about something.