I've found that when I'm feeling really depressed, which usually happens in conjunction with feeling worse head-wise, that taking even a short walk in the woods or just getting out in nature really helps. If you can't get out, perhaps just listening to some nature sounds or looking at pictures of nature might mimic the effect. When I was feeling my worst I would often just listen to this "Sounds of the Grand Canyon" CD i got when I was a kid that has nature sounds (e.g. running water, birds, wind, rain, etc.) mixed in with new age-y piano music.

I'll second SuperElectric's suggestion of having more interpersonal interactions. I know this prohibitive for many because of symptoms, but if you can manage something it could be really helpful. Personally, when I took a leave of absence from school and moved back in with my parents to focus on recovery, after a few months of not doing much of anything except appointments, I started volunteer coaching for my old high school cross country team. Running had been my world for high school and college, and being around that atmosphere, and especially interacting with people eventually did more than any treatment I've had to make me feel a little more like myself.

It wasn't always easy, and I started off only going to practice a couple times a week and often felt worse afterwards, but it turned into something that really helped stave off deep depression/despair because it expanded my world beyond worrying about the concussion. I also went through a distressing breakup shortly before I went on leave, which was another major source of negative thinking, and coaching helped me deal with that somewhat better. I really hope that if possible you might be able to find something similar, whatever that might be for you given your symptoms. I hope things start turning around sooner, and know that even if it feels like no one understands, people on this board, in the limited capacity that something like this allows, do understand and care.