"Organization" refers to both an executive function and a set of skills, i think, and if its hard to organize its hard to learn organizing skills too .. theres a lot of different methods and ideas for organizing and it can get overhwelming. pick what works best for you - if you're not sure, then pick a method randomly and stick it out a while, get a feel for it.

i like the book "add-friendly ways to organize your life" but halfway thru i was overwhelmed by all the tips on how not to get overwhelmed (whoops!) since you ahve problems with decluttering right now, heres what i thought was most useful.

cleaning whats already there is a long term goal, so split it into steps. i suggest one room at a time, for 30mins at a time. set a timer and when it goes off, step back, stretch. decide if you're going to do another 30 minutes, and if so, make sure you're focusing on the right task. rinse, repeat

you can prioritize rooms based on how long it will take, how important having that room clean is, or simply start at the entrance room and work your way around.

keep three boxes with you labelled 'give away', 'throw away', and 'transport'. definitely label them. the first two are as labelled, the third is for any item which belongs in a different room. that way you really can take it one room at a time and not break your concentration.

if you find yourself retreating to minor tasks when you need to address the big picture, the timer & stretching can help break you out of what you're doing so you might go, "wait a minute, why am i organizing my thread spools based on color and size?" When you aren't sure where to go next, narrow the focus by picking visually & without too much thought. "the bed" is as good to start as "the dresser", so whichever you saw first is first. (visual is easier to keep track of than something more abstract like "things i borrowed from people")

if you can, have a friend or family with you. they don't even have to clean - having someone there to keep you company can keep you energized and focused too. other ways to make cleaning more energizing are listening to (especially singing along with) music, or planning a fun task to do afterwards

anyways theres a bunch of tips on organizing mail and incoming items, and keeping a out box next to door for things like packages you need to mail, and stuff like that.. but it might be hard to figure everything out at once. so you could choose between "decluttering", "preventing clutter", and "maintaining clutter" to work on at first.

And keep in mind if its a complete nightmare, you can always hire a professional organizer. Good luck