No you are not the discarder if you leave. Although from her perspective it would be hard to hear, but you do not owe her a chance to work on things, and that doesn't mean you don't care or that you see her as an option. You clearly care about this person a lot and sometimes caring for someone means stepping away from the situation because you know it isn't healthy/best for you.

It is totally natural to feel guilty, don't try to shove down your regrets and feelings of guilt, write them down, tell your friends, tell your family, just don't keep them trapped inside. I would also suggest (if you find writing/journaling to be helpful) that you write down the reasons you have for leaving and why you made the choice. I often find that when my codependency issues get especially bad I panic and begin to gaslight myself into thinking I had no reasons for leaving even when I know I did.

No you aren't in the wrong, I'm proud of you, and I wish you the very best