If I remember correctly, six months ago, you dated her for a few months, and you saw her as an LTR girl.
Then you told her about your prison time and that you will kill a man if you need to. From then on, it was down the hill, and she told you she didn't want to date you.
Somehow, you stayed FWB.
She made her mind a long time ago.
But you're playing it like exclusively is on your card, and you'll give it to her if she deserves it.
But the truth is that you can't give her anything. She didn't want you as an LTR guy then and still doesn't want you now.
You can't give or offer her what is not yours to give ( exclusively).
I think you found a girl who doesn't see you as LTR material, which may hurt your Ego to the point that now you must have her.