Friend issues

I don't really know what I want to achieve from writing this, probably to help myself feel better or understand the situation. Im not going to name the people involved I will just tell my story. So basically me and my friend have been friends for 2 years, knew each other in college and that's where we meet friends X she is extremely childish, we were constantly dragged into her boyfriend drama and on nights out she went as far as to slap my friends boyfriend twice and punch him in the stomach. She's done lots of things that I'd spend all day writing this but I won't. So anyway another friend of ours invited me my friend and X to her night out and were planning on getting taxi with them. However me and friend could not afford to pay £20 quid each and said we'd get train and meet them there? Fair enough enough right? Well no my other friend who runs the party was upset and wanted us to come in the taxi and we said we can't afford it and friend X attacked us! She began messaging me firstly calling me a "nasty cow" "everyone hates you" "no wonder you have no friends" "I feel sorry for your boyfriend going out with a disease like you" so on and so on. She attacked my other friend and we ended up not going this night out and cutting them out of our lives for good. Or so I thought. I cannot get over the hurt, those two spread rumours about us in our local town, tried to attack us on nights out everything. Anyway, recently I saw on social friend had met up with X and they are best friends now! They comment on each other's status, my friend even put in response "glad to see you sick of holding grudges and being silly" 
I feel humiliated! I thought we stood together and not let them wriggle back into our lives, my friend has betrayed me, all the slagging off she did about X and now this! She's friends?! FRIENDS?! I can't, I haven't spoken to my friend for weeks because of this. How can I get over it? My other friends just tell me to leave her too it or ignore her, but I just want to know why she seems to be so desperate for friends!!!! What do I do? Continue to ignore her? Cut her out of my life? So so angry and feel betrayed.