I would just set her free. If there are no rescue groups in your area, and the cat is already used to living free, you can just go ahead and let her go. You already did your part: You Spayed her, which is so important!!! She managed to survive before. Cats are smart, and her wild nature is bigger than her "pet" nature.
You would not be dumping her. You'd be giving her what she deserves: Her Life Back.
This is tied to the other post you put up-
Your best bet is to contact a foster group. See if there is one in the area. They people in these groups are willing and able to put in the work needed to cope with a feral.
Keep in mind this is no slur on you. Working with ferals is HARD, and takes a lot of time and energy that people sometimes just don't have.
Admitting you aren't able to do this is a responsible action on your part (if this turns out to be your decision).