Your medium is way too wet and it's very easy to overwater a plant in such a big pot.
I would make a very light, airy mix of soil and repot her into a smaller pot and let the soil in this pot dry out as she recovers.
Once she gets going again, you can put her back in this one, but I would also add perlite or something to aerate the medium more.
You can start a new plant, but if you don't understand what happened, then you are likely to have the same problem again.
Fixing your mistakes teaches you more than just chucking them out any time you do something wrong.