Was on pristiq (desvenlafaxine) 6 months ago, suddenly one day got extremely tired, been that way ever sense- did it mess up chemicals in my brain? i stopped taking it

Not likely: You've been having a hard time trying to pin down your health problem. That's stressful and so can exhaust you severely. It is a common side effect but it not likely to still be connected to it. Consider a Health Psychologist to guide you through Dx/Tx/frustration of medical mysteries. You can learn how to understand your body's reactions to stress and get some control over it. Best!