girlfriend wants me to go on a double date with her friend that I hate. What should I do?

Anonymous
Long story short, she was supposed to have a girls night with this girl. Now her friend wants to have a double date with her new "boyfriend", I don't want to go. This girl sucks, she is selfish, has stolen from me, and even tried to get my girlfriend to leave me. She doesn't take care of her son, and is just an overall terrible person. My girlfriend even stopped hanging out with her for a long time. The only reason my girlfriend hangs out with her is because she doesn't have any other friends. I have given this girl so many chances, and she has ruined all of them.

I told my girlfriend I don't want to go, and she should just have a girls night with her. But now she is mad and saying if I don't go she won't go to an event this weekend that we already planned with my mom. My girlfriend backs out of shit all the time, and now she is giving me shit about it. She missed my mom's bday just a while ago cause she didn't "feel like going", shouldn't I be able to do the same?

I feel like I don't owe her friend anything. I really don't want to waste my Friday night with them. I feel like it is selfish to make me hang out with this girl considering our past interactions. Her friend doesn't even like me, she treats me like a third wheel when we do hang out.

Should I just go? Stick with my "no". Or maybe even dump my girlfriend for always bailing on me, and then getting mad when I do the same? It really pisses me off that she is using this time with my mom against me.
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Also, we have been dating for almost 2 years. We live together for the last year or so. I am 29, she is 27.
girlfriend wants me to go on a double date with her friend that I hate. What should I do?
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