I know and I have. But the alternative is to be ruthless and cut her out of my life forever. And forever is not an exaggeration. For a number of reasons, after next year I would literally never see this person again in my life if we don't patch things up in the upcoming months. It upsets me that I might not even get a chance to see her in the upcoming months; that forever might have already started.
Believe me, I've had to cut people out of my life before and once they are out they are never, ever in again. I had an ex basically offer me sex and I turned a polite, but cold shoulder. This is what's keeping me from crossing the Rubicon, knowing that I have a limited window which can never be opened again.
In essence, this is the whole reason why I came to this part of the forum: To ask ISFJ's if this thing could be turned around. If it can't, I just have to move on.
If you want it to turn around bad enough, I believe it can be turned around. Keep in mind, ISFJ's are extremely forgiving (especially with people they care about), understanding, and kind people. I would say your chances are pretty high, but then I remember what you wrote about having sex with her best friend. WHY DID YOU DO THAT? haha. That was a rhetorical question btw.
I hope things work out for the best!