Well done OP on tackling this! This has turned into a mammoth post so I apologise!
Maybe borrowing a concept from other fields (sport/habit-building) will help. We can't rely on motivation to do anything for a long chunk of time! Motivation is a short-lived fuel; it's there to get us started, but after that, we have to rely on a couple of things: discipline; habit, and deferred gratification, knowing the end result WILL be worth it even though it doesn't feel like it!
Think of this like the gym: you've been going 4-5 days a week solidly, and you felt great, but now the novelty has worn off completely, and you have no motivation, so it's all about those other three things.
Could you set yourself two slots a week that are decluttering time? And that's it? It's a habit, but it's manageable, so it won't feel like "Oh I should be doing this but it's so unfair, it's Sunday!". Do it on say a Tuesday morning before work for an hour, and then a Thursday evening, or just two hours on a Sunday. You still deserve relaxation time! The habit will be there, it will be less overwhelming and you might have less guilt.
Can you write down on a lovely piece of paper, five benefits of working through this process? What will you gain by doing it? Look at that when it feels too much.
On the wardrobe thing, could I make a suggestion? Not sure how feasible, but if you have a spare room... Set aside two hours. Completely empty your wardrobe, and have a few boxes/bin liners/containers for different categories for EVERYTHING! Work clothes/casual/underwear/accessories. Take a tiny capsule wardrobe of your favourite/most-worn things, back into the wardrobe (depending on your needs, eg if you need more work clothes because you have professional job). Everything else goes in the spare room for now. Once a week, take an hour or two to just do one of those spare room boxes/bags, and only put back in what you love. Anything else goes Donate/Recycle/Regift/Sell. Repeat until you've done all categories. That's how I would do it if I had your wardrobe problem; it's too much taking away from something very full, bit by bit, but building something from an empty wardrobe, you'll feel psychologically better and will be able to be more ruthless.
Good luck and keep plodding on! It will be so worth it and you should be really proud 