Out of your choices, I would just stop contacting her and leave the hurt and anger behind.
Like PP's said, I doubt a letter would do anything but leave you hurting more when she doesn't respond. Write it if you think it will help you move on, but burn it. Don't send it.
So don't cut her off, just let her make any contact. If she does and you want to see/talk to her, you can. If she doesn't, then let it be.
I've had a few of these one sided friendships and they are hard work. In the end, I stopped contact and making plans. Some realised and made more effort, some slipped away. But I believe that those that slipped away weren't real friends to begin with. Friends should be fun, not hard work.