Self awareness is the first step. You already know why you cannot finish a long fast. So it should be easy. Posting your affirmations daily may help you, but also don't count days seriously. Just focus more and more on how your body feels. Your body will have two responses, one I feel terrible because....rash, headache, etc. Second, I feel great because....I lost 5lbs, my skin looks radiant, my body is throwing out all junk, etc. If you are really in tuned with your body, you will be situations where your favorite food is on the kitchen counter or someone offered you a goody, your body will reject it. At that moment it is very important to listen to your body, the voice from your gut.
Focus on things that make you happy while on fast. Resting and sunbathing is equally important.