Look after yourself pbat, there are days like that, but they come and go, just as surely as better ones do. Give yourself a break- music, films, reading, chat a friend or relative on the phone- whatever is a distraction for you.
I've had days exactly like you describe. You keep dwelling on the negatives, or sometimes it's nothing you can pinpoint, just an absence of happiness. It's a sticky mess, hard to shake off, and it feels like it's pulling you down, and it feels like your thought processes are all over the shop.
What works for me, now it did take a while, is to try and train your mind to sectors. You can't just block it all out, and trying that, or trying to pretend it's not there, takes a hell of a lot of energy. Give yourself 20 minutes every so often where you think about it: analyse it, jot down lists, what you think is wrong, what you think are problems, what you think are weaknesses: whatever is on your mind for that 20 minutes, then put the brakes on, and take time to clear your mind. Listen to music: your favourites, old memories- it doesn't have to be happy-clappy, or eye of the tiger stuff. Chat someone- a friend, relative, whoever. Watch a film or a documentary. Read something. This is 'you' time. Later on, go back in your mind and how you feel. It's like a boxing match: you train, you spar, you fight a round, you take a break. Or you can use a football league analogy and prepare for a match.
What's important is that, over time, you are calling the shots, and you are dictating the terms of when you are engaging. That feeling you described today, I know that. It feels overwhelming, stifling, it's wearing you down in a war of attrition in your mind, and that is hugely energy-sapping. But it doesn't have to be played under those terms. I'm 3 years on from my last bad bout of depression. It's not a bed of roses since, and it's not going to make a rags to riches film anytime soon, but I have only lost a very few rounds since. And I am getting to the stage that I'm pretty confident it will never have the same hold on me again. And that's a good feeling. It's there for you, too, and you'll get there.
One of the most significant lines I picked up on my journey was 'How do you eat an elephant? One bite at a time'.
You take care, and the very best of luck to you.
There will be better days ahead.