I've been having the urge to text my best friend, but I made a promise that I wouldn't.
Why would you make a promise to not text her?
We got into a fight and kind of ended our friendship and she made me promise never to text her again.
Given the situation I think it's understandable you want to text your best friend. You guys ended a friendship over an argument and I think that's a promise made in anger so you shouldn't take it seriously. You miss her and she likely misses you two so you should try to make up.
Am I bad for texting her now that I promised not to?
I wouldn't feel too bad about it, technically you did break a promise, but if you play this right it may be good. Just calmly Explained that you're sorry for what happened and want to fix things.