You haven't made a mistake. I can see that you are REALLY trying and that you love your dog. Asking questions and committing to do more isn't easy.
I thought I made a mistake too when my dog was younger. Even though my breeder told me she was low-moderate drive and energy level (which I see now at 2) she was NOTHING like this at 6 months old. She was the holy terror and I was crying ALL the time because I was so tired and felt like I had no time to do anything else and was always questioning whether I was doing her right by keeping her.
This period will come to pass! Just tough it out for a few months (which I know sounds IMPOSSIBLE right now, but it will go quickly).
An alternative to tethering is to get a lightweight long nylon leash and have your dog drag it in the house. That way, you can grab the dog when it is doing something bad without enforcing the bad behaviour by chasing your dog through the house.
Tethering was difficult for me as well. I had to really tire the dog out before it would work. At that point, dog would probably fall asleep if I left her alone anyways.
I crated the same amount of time as you. They do sleep for MANY MANY hours a day if left alone, so I wouldn't feel too bad about if if there is no workable alternative for you. I got the biggest crate I could buy, and that was the extent of me changing the situation.
You are doing so much more than many would do for their dogs with the HOURS of exercise and patience you have put in. This WILL pay off. At 2, my dog is an angel!! Yours will be too, give you both some more time.