Your title seemed to indicate that you were on your 40th day of the Master Cleanse. Is that right? If so, that might explain why you're feeling tired. That's a long time to be on a such a restricted cleanse. At 40 days you're probably experiencing numerous nutritional deficiencies.
It's not that the Master Cleanse can't be beneficial at times. But it will deplete and weaken you if used excessively.
Also, bear in mind that cleanses are for cleansing. If you truly have additional excess body fat to shed, other measures are needed, not just cleansing.
Best wishes with your health.