I'm sorry you're having such a hard time, especially since it's your first time with the hobby. If I hadn't started a planted aquarium using her method, I think I would have become disheartened and given up pretty quickly. The more pieces of the ecosystem that you have to control/keep balanced, the easier it is for things to go out of balance.
imho, the substrate and lighting are the determining factors as to how much you have to supplement co2 and nutrients.
With the Walstad method, as I understand it, the soil acts as a mechanism to turn the fish food and waste into nutrients for the plants. It's actually better to "overfeed" the fish as she calls it "feeding the plants". The plants help to clean the water and remove co2. However, you have to low-light (1-2 wpg) and a good amount of plants for it to work in an aquarium, and be sure to not overstock fish. She says at one point "it's better to over stock in plants, than to over stock in fish".
The worst thing that ever occurred using her method was during the first month of setting up the 75g at school. One day out of nowhere the entire tank was green, the water was green I mean, so much that you couldn't see anything inside. I left the lights off for a couple of days and on the third day, the entire tank was clear water again... just like that.
It really was much easier, and cheaper. You do have to have the right expectations... like you won't be able to grow lush groundcovers like hc, imho. But, I enjoyed the tanks I had and the plants thrived, the fish were happy and it just worked. (In hindsight, I'm not sure how much Boulder's city water may have contributed to the success. It was really clean and pure, compared to many other places. Our in-laws always enjoyed our coffee, saying the water made the difference.)
Another consideration is if you try this method, you'd have to revamp the substrate. It would be a redo.
Maybe try reading the book while giving the ferts a go (since you have them) and then decide later. (the majority of the book is mostly the science behind why each component of the tank works, complete with her experiments and data tables. If you're like me, I just skipped to the 'how' at the end and followed the instructions.
You could also setup a second tank using the Walstad method, transfer in a small bio load of fish, and slowly transfer over the other fish until it's empty then sell the initial setup.
Woah... sorry that was so long! But, I hope it's somewhat helpful. Let me know if you want the book and I'll get it out to you.