It's Saturday. You slept in. And you are still shattered.

Why are you so tired?

1. You may well be not giving yourself enough recovery time. Your job is analogous to the gym; you can't keep lifting heavy weights without some decent rest and re-hydration. It's just the same with a demanding job: especially if you do a lot of travel/driving. Build in recovery time, now.
2. Not enough sleep. If you are reliant on an alarm to wake you, by definition you are not getting enough sleep. Clear your sleep debt.
3. Poor nutrition. There's fuel (cola+crisps). There's food (curry+rice). And there's nutrition (grilled fish/meat+green vegetables). Change the mix to ensure more of the latter and less of the former.
4. Mental fatigue: stress. Get out of the 'worry' mind-set by (1) associating with positive people outside your work circle (2) reading (3) listening to music (4) getting out into nature...
5. Healing crisis. Sometimes when we start to look after ourself, we feel worse before we feel better, an effect known as the healing crisis. Keep in touch with your doctor, take it slowly and give it time.