It could be just chemical or just environment, but I find for myself it's a combination of both. Find a leg up anyway you can to budge the depression - which for me, leaves me in a place where no language can reach. Even it's just a 10 minute walk outside. Try medication if you haven't already - if you have and it doesn't work, try another one. Mainly, keep trying. The terrible valleys don't stay forever. I have to accept I always have and always will have depression - even if it's just "average" - low grade depression or at least have a tendency for it. Finding a way to live with it and finding ways to mitigate it are what I search for.