When it comes to motivation, it's almost as though you have to trick your mind a little too sometimes. Perception is everything! I really like what one of the earlier posters said about looking at this journey from a perspective of adding new habits rather than subtracting things or depriving yourself.
And its great that you motivated yourself to exercise for 20 minutes! Even on the days when you put in 5 minutes, instead of feeling guilty, remind yourself that you at least did SOMETHING toward your goals. I really think that its all about doing something, however small it may seem, every day to help us achieve our goals. How we "perceive" our efforts goes a long way to helping keep us on track.
Believe me, I suck at motivating myself too! I have a long road ahead of me, and starting is always the hardest part. Good for you for reaching out and finding a way to inspire yourself.