I know exactly how you feel, had a similar problem myself. Go to your local citizens advice, make an appointment - they are great used to filling in the forms and saying the things that matter, you will get it backdated, probably need a doctors note, but not sure how far they backdate these days, also the Govt dept that deals with this is not as hostile as they were years ago I found them very understanding, they deal with this type of thing daily. Citizens Advice would be my first step.
Hope this helps, depression is a pig, had it for years and now have a chronic health condition to deal with as well, and its getting to the worse time of the year.
If it helps I joined a local walking group (not really mcuh of a walker) they were great all helpful and friendly see if anything runs near to you. Once you make the break and get out, you will feel better even if its only for a couple of hours.
Good Luck, I know you can beat the black dog!