Struggling with somebody sabotaging my social life

Hello
I am new to the forums.
I have been having difficulty dealing with a nasty woman who has been causing trouble in my friendships for the last year.
We were friends for awhile before I saw through her and realised she was using me and going around me to get close to my close friends and excluding me.
I tried to fix the problem by talking it through with her but she has continued to undermine me since.
She is a part of my larger social circle and every time I have tried to avoid her by joining different groups she has followed me and wooed my new friends on facebook and pushed me out.
A few of my close friends see what she has been doing but don't stick up for me as they are scared of being the next target or feeling uncomfortable. Others think I am being paranoid because she is so subtle they don't see what she is up to.
She has now managed to make friends with a male friend of mine who is very important to me and has told him something about me that has caused him to turn on me. I feel like she has been gaslighting me. It has caused me so much anxiety that it has triggered my auto immune disease and I have been off work all week. I am starting to doubt myself.
Any advice would be very welcome...