Why is my girlfriend acting like this?

[David]

hey.
So a few weeks ago I was on facebook and i saw that my girlfriend had been commenting on her ex boyfriends profile picture saying: omg your so hot, so sexy and you've got the muscles.
I dumped her because of it and when i asked her why she said those things she couldnt give me a proper answer. I landed up going back out with her a week after though, by then she had explained herself. its been a month now and yesterday i decided to meet up with my ex girlfriend, we met up and had an awesome time together but she told me she still has feelings for me, i still have feelings for her... we both want to get back together but im in a relationship, i couldnt betray my girlfriend so i was going to quit our relationship...and thats when my bestfriend tells me that my current girlfriend knows im going to dump her and that she will ask my bestfriend to date her! MY BESTFRIEND! DOES SHE NOT CARE! HE'S MY BESTFRIEND! How do I dump her without being cocky yet i can be little bit harsh on her? what do i say to her when i dump her? any help will be appreciated?

[Lucy]

You do nothing. I have to admit when I read this i thought that it was a pretty dick move to meet up with your ex. I think i understand why she's doing this and I would try and stop her soon. But seriously I have to say I'm rooting for her. Sorry just saying that wow it's hard not to go on a itch rant about this.

[Nosher rides again]

Just slap the bitch and tell her she stinks like Grimsby on a summer afternoon.