First: Congrats, you've got a great space you should be proud of
Second: Don't feel bad, you're literally fighting a fundamental Law of Nature that is as basic and constant as gravity. Call it entropy, call it "nature abhors a vacuum" or whatever else you want....but space doesn't stay empty by magic....it takes work, often significant work and that is why there are companies, consultants, authors and an entire eco-system of folks making money on organization related items
So here is my brief take:
Start small, improve a defined area, take that success and move to the next area. Rinse and repeat until done
How do you start...that's harder
In the 5S factory methodology made famous by Toyota Motors, the first "S" roughly translates as "sort" which seems applicable to both garages and factories. No point in spending effort storing things you don't need, so first identify all the stuff that isn't going to kept around.
Based on the collection of things, I'm expecting it's not all your stuff and other family members or friends may have some ownership of the situation, it also means to be successful they must have ownership of the solution. So what I've seen work for others in similar boat is a modified version of what factories do:
Make (3) "Piles"....and by piles it doesn't have to be a pile but a pallet, box, crate or at bare minimum a physical, taped out square on the concrete...there should be absolutely 0 ability to argue if something was in the "pile" or not
Then label(again 0 ability to argue which pile is which)
-Your's and keeping
-Your's and disposing
-Other folk's stuff
Once you get your stuff from that identified area you're starting with sorted(reminder it's darkest before the dawn), give everyone else who has "stuff" a reasonable period of time(reasonable is your call) and make sure they know that if it's still in "the pile" at the end of the time it's getting disposed of at your earliest convenience...then stick to that. If it leaves the pile it must find a new home, NOT in the building and preferably someplace "appropriate" so it doesn't just form a new pile somewhere else(garage...basement....etc)
Once you're down to just stuff being kept, it's time to make homes for the stuff....echo the prior comment of pallet racking being cost effective storage. You've got sidewall height....if possible make use of it...just remember that vertical height like racking gives is difficult to easily use without a forklift....so are things going up high really worth keeping??? Seasonal stuff is one thing....but that box of X that "may come in handy" but has 5 years worth of dust....should it find a new home instead? Selling stuff that's been collecting dust is a great way to help fund the storage solutions as well so this can be a cash flow positive project as well!
End of the cycle for that "area" you should have a much cleaner corner and some additional energy to keep tackling the next spot...but it's not magic, it takes work. It takes work to make the spot for things to go, it takes more work to make spots for things to go that are logical and easy to use and it takes work to put stuff away in the correct spot, even when they are well thought out spots. But once you have that well defined location, with only the items that are actually needed, and everyone is proud of the area...it does seem like less work to keep it there as you're generally enjoying the time spent on and in the space much more!
