Caffeine, like nicotine, is an addictive legal drug, just not as addictive as nicotine. The body has its own natural chemicals and when you introduce a chemical replacement or add more chemicals than the body needs, the body makes adjustments to its production.
Just as it takes a while for the body to react to overdosing, in this case caffeine, it takes a while for the body to return to natural production. It is said it takes 30 days to break a habit, maybe that's partly due to the body making adjustments to try to get back to normal.
The problem you're facing now isn't ambition, it's energy. You can ramp that up by simple exercise. Take a walk, do some Yoga, anything to get the blood flowing. The more intense the exercise, the more energy you'll gain from it.
Julie