I don't know if I can stay true to her
That doesn't sound good. Will you abandon her instead?
Yes. I'm sure I will abandon her before long
You should do your best to stay involved with her if you love her. To abandon someone after having been in a relationship for so long can seem very rude.
But what else can I do? I'm not feeling any emotions anymore and it seems like my brain has shut down.
It sounds like you need to have a sit down with her and talk things through. If you truly don't have emotions for her any longer than  be honest and tell her. She will just have to accept this reality and you both can move on and start the healing process.