Fatigue from depression - How bad can it get?

I've been battling major depression for almost a decade now. The single biggest thing that keeps me down is this soul crushing fatigue that I feel day in day out and can never shake off. I have constant brain fog, lethargy and poor concentration. My brain feels so slow these days that it's hard to hold a basic conversation, with me forgetting words and stuttering embarrassingly bad grammar.I've tried all the things that everyone recommends. Sleeping at the same time every night and getting plenty of sleep. Frequent exercise. Eating right. Yet nothing changes. I'm on Wellbutrin now, which is known to perk people up a bit, but I I don't feel it at all. My sleep is unrefreshing and I feel starved of it every day, no matter how much of it I get. I've been averaging 11+ hours a night lately. I know that oversleeping makes people feel bad too, but if I get the "normal" 7-8 hours, I feel way worse, like I just pulled an all nighter. I made a thread here a while back asking how much people sleep and the replies I got were a somewhat normal amount of hours.I feel like if I wasn't so completely exhausted I could actually deal with my other issues to some extent and at least have some semblance of a life. Every day I ask myself if I have some disease/something wrong with my body, or if it's just the depression. I've had so many tests done over the years that have all come back fine. Yet there are all of these somatic syndromes out there that cause these same symptoms.For the past year and a half, I've felt like something else is causing my fatigue, and that complete lack of energy to function is what is making me depressed, and not depression causing the fatigue.I figured that if others here can relate to this, maybe depression is really the cause and I am just in denial. The family doctor that I started seeing in January thinks so. I just don't know what to think at this point.