Choose a small room first, like the bathroom. Clean it up, you don't need fancy cleaning agents just cheap bleach and toilet roll/old t-shirt.
Put all rubbish outside in bins immediately/take it to a tip if you can. Even stuff that isn't rubbish - take the contents down to bare minimum, it's easier that way to clean again later on.
One room at a time, rubbish out, throw out everything you do not need, all knickknacks that just get in the way cluttering up surfaces, window sills, cabinets etc. It's hard to do, but it really does make cleaning up easier in the long run. You don't need that stuff.
Soiled carpet? Throw it out, better with clean floorboards than carpet that will just bring you down and make you feel bad. You can buy a super cheap carpet down the line and it will still feel glorious and lovely.
Decorate? Don't worry about it, don't think about it. Declutter and clean, that will change the look of your house immediately and you will feel better.
I feel for you, it's such a vicious cycle - you sit there and feel down and powerless with no energy to clean, but the sheer amount of cleaning makes you feel down and powerless.
You might find that once the house is tidy/clean (doesn't have to be perfect show home remember!) that you gain more energy just because you're feeling less upset by it.
It doesn't have to be this way forever, I promise. You're not doomed to live like this always, trust me.