This thread title intrigued me, so I'm sorry for participating in the vegan section. I know, I such a rebel.
My mood goes through periods of intense instability, and I have been diagnosed with depression twice in the past (both before I was a vegetarian). Sometimes these old feelings of hopelessness, sadness, insecurity, and meaninglessness return if something happens to trigger them. I always question my vegetarianism then. When you're thinking "What's the point?", as usually do of everything in a dark period, one of the first things I feel like doing is going down to McDonald's and tearing through a double cheeseburger. (When I was an omnivore, I only liked the crappy, processed meat).
Alternatively, I can feel self-destructive in these dark periods. I haven't physically injured myself since early high school, but feelings of wanting to deconstruct or hurt myself remain. They are just more subtle. Vegetarianism has become part of my identity. By forsaking that, I would be committing some self-injury in a sense. I don't know why I feel this way when I'm sad. Maybe it's just a way to get out of your own skin or release some pressure.
All that said, I have never done it. Not even once, in secret in the dark of the night when nobody was around. Maybe part of me knows that it will just make me feel like crap later. Maybe it's because animals (particularly my own dog and cat) are such a source of peace and calm for me. Seeing and interacting with animals always makes me feel good. Eating meat again would compromise this effect, I think. My relationship with the rest of the animal kingdom would not be so positive if I ate them. Maybe, too, it's just because I know cows don't understand human depression. "I'm sorry for killing you, I was sad" is not going to alleviate their suffering. I try to avoid lashing out at other people when I'm upset. Animals are even less-deserving of this lashing-out.
I'm sorry you're going through a rough patch, Labile Lotus. :