Why would my "friend" constantly try to sabotage me?

Anonymous
I never really have that much success with men because I am shy and in the past I was kind of overweight and not that attractive. She knows about my failure rate with men. She used to try to help me meet men. Ever since last May I've been working out as much as I can. I figured I should make myself more attractive looking. I've also been working on my shyness. The last few months any time I talk to men she sabotages any attempt I make with them.

Last night I was talking to this really great guy. He asked for my number. She came over and told him that he doesn't want to go out with me. She said I am a nasty skank and the city bicycle and he wouldn't want to catch anything from me. I'm still a virgin. I have never had sex or anything.

I don't even think I want to be friends with her anymore. I have asked her why she keeps doing this to me several times. She used to be such a good friend. She says it's just a joke and I am taking it too seriously. I told her that I'm just finally having success with men and she ruins it every time, and I want her to stop. She said she would, but then she pulled that stunt last night.

How can I get her to realize that I don't find this a joking matter. I'm 28 years old, I'd like to get serious with guys. Also, she's 36 so you'd think she'd be too old for these kind of jokes.
Updates
+1 y
Thanks to the people who have answered this. She did it to me again on the weekend. I saw her today at lunch and told her everything I was feeling and how sick of it I was. I told her the friendship was over. She left upset but I've had about three texts begging me not to end the friendship and that I was being childish. I told her I'd told her enough times I didn't like what she was doing. She told me she wants to prove it to me she'll stop. I said no.
Why would my "friend" constantly try to sabotage me?
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