IMO your "bad" self (the one who self-harms and wants you to suffer) "hates" her now because of that promise and because of how difficult it is to keep it. That's why you want to cut yourself in front of her, it's sort of a punishment. "See? I will cut myself to death, I CAN and I WILL and you are obliged to witness this! You wont stop me!"

I know quitting self-harm is hard as hell, but take it one step at a time.
Try just to reduce cutting at first and once you succeed, you can try to stop completely.

Don't pressure yourself too hard. She already knows that you might not be able to keep your promise, she's not gonna be mad at you for this. She only wants you to try.