@Changeymcchangeychange Absolutely no judgement from me. I have been there and it took me two years to finally get rid of the CF in my life. They simply wouldn't take no for an answer. Only now, I realise I was being guilt tripped, blackmailed, gaslit with every no I said.
I don't know how your "friend" will respond, but try saying no again and offer no explanations other than "because I don't want to". If she ignores it, simply don't do the favour at all - be out somewhere, be unavailable. If you have already said no umpteen times, then nothing you can say or do is going to make a difference and you need to get her out of your life completely.
Stop answering her messages, mute her or block her. If I can do it, so can you. She may try to worm her way back to your life again, but by that point, you will have had the space to think more clearly and realise that she is a toxic creature.
You say she is your only friend, but once she is out of your life, you will gain a renewed respect for yourself and your self confidence will soar. She is not a "friend" worth having because there is no loyalty, no support, no respect or regard for your well being. She's a leech and she will continue to drain you as long as you relent to her demands.
Those posters who say things like "grow up", "grow a pair", "don't be a victim" are not being helpful in any way shape or form, and don't seem to have any conception what coercive control and psychological abuse does to you.
Wishing you the best of luck op. You need to do this for the sake of your own mental health and your fiance who DOES care about you.