You want to "feel mentally and physically healthier" yet you are planning on depriving your body of essential nutrients it needs... You think drinking only water for 15+ days will bring you "Mental Clarity" ???
I would recommend eating a least 1 blender meal per day in which you consume a salubrious amount of fruits and vegetables... For example in your blender put; Spinach, Kale, Beets, Blackberries, Strawberries, Blueberries,1 x Banana and maybe some carrots / half a cucumber.
Try this technique instead of fasting on only water and do not be surprised when you not only experience enhanced mental clarity, but your physical health will also greatly improve.
This is a much healthier technique to stick to and once you go back to your more normal eating habits, start to substitute unhealthy meals/cravings for healthy spinach/kale/fruit shakes. Give it 20 minutes or so after you consume one and unhealthy binge cravings usually subside.
Best of luck m8