We are way more than sacks of H2O.
What you need to do (diet-wise) is to:
1. Stop intoxicating yourself with bad chemicals found in Sodas, Sweets, almost all processed food and all those thing famous for being unhealthy (including everything marked as "light" !)
2. BUT MOST IMPORTANTLY: Because we are more than sacks of H2O, you need VITAMINS, MINERALS, Co-factors and protective bio-molecules (which activate protective genes).
Just for those who don´t believe or think I´m vegan or w/e. We come from Monkeys, they live in rainforests, and eat mostly fruits and leaves. Because of that diet, out ancestor Monkeys lost the genes for producing certain vitamins and co factors (why have that info and waste proteins and nutrients into making something we already get a lot by just eating right??). So pretty much all monkeys from the hominidae genus (which include us) lack those vits and cofactors.
Shot story long: -Vits/Minerals/Cofactors deficiency ->pathways gone wrong->deficience/wrong Neurotransmitters->affect physical and mental state.
So instead of drinking just H20, try making green tea and sweet it with honey. Do some smoothing with a great verity of fruits and combine it with taking omega3/fish oil, you will feel a lot better, believe me. I have been where you were, before I started studying Molecular Biotechnology.
Also, because you said you have been exercising for a while, you need those nutrients even more, and maybe the exercise led to an increase in the need of those nutrients (because the ones you actually consume go to the muscles) leaving your nervous system vulnerable-