I apologise in advance of you reading this post for my ability to ramble on and off topic. But I hope some of you guys can give me some pointers to what to do in these situations.

Last night I went out with my Cousin. Just the two of us.

First thing. I got this girl, a 6, on Facebook, but I was more into her mate. Her friend was an 8. Nice good looking girl with blonde hair and a cute smile. I was dancing a little with her whilst my cousin was dancing with the 6. Then the 6 started laughing at my t shirt and moved over and started talking to me. The 8 then danced around us for a minute or so but then went off with some of her other mates to a different part of the dance floor. The 6 carried on talking but I couldn't hear her, so I passed her my phone and opened the 'notes app' so she could type. Turns out she half knew me and was friends with a girl who's older sister I was in School with. The 6 then opened my Facebook and added her self, after she found out what I do as a job and things "I really want to do that". But this is all great, but I have no interest in her, I liked her mate and I kind of blew that? What should I of done and how could I of rescued it.

We saw one of the girls we know, she likes my cousin but he has a girlfriend. But her mate started giving me the eyes. We were on the dance floor, so I started to dance next to her. She came closer and gave me more eye contact so I danced with her. She was about a 7 but she thought she was a 9 or a 10.
She started to dance quite erotically with me, I let her grind on one of my legs. I think she didn't want her friend to see to much, so she took my hand and led me closer to the bar, in a corner of the room where her friend couldn't see.
She continued to dance and then came real close to my lips then went to my ear and said, and I quote, "Buy me a drink and I will 'neck' you". I thought, what a slutty thing to say. My response was "Neck me then and I will think about buying you a drink". I know the whole rule of don't buy girls drinks etc. So we started kissing, probably for about 5 minutes maybe longer. I could taste she was a smoker.
I bought her a Vodka and Lemonade. Student nights are cheap so I wasn't to bothered. But I made her promise she would get me the next one. We continued to kiss.
After the drinks I said right your turn. I will have an Orange VK and Apple VK. That works out at the same price as what I bought for her. She only bought me the Orange VK.
We went to the floor above. Little bit more kissing and dancing. Then I spotted a girl wearing a "pub golf" outfit with the golfer hat visor. So I said I want the hat for some pictures. She was like why? I said it would be funny. So I took her over to that girl. I started talking asking to borrow the hat for some pictures. She agreed. While I was having my picture taken by that other girl in her hat, the girl I was with started to chat to some guy. It looked a little bit like she was friends with him. When I looked again, she was kissing him. So I gave the other girl her hat back and went off to find my cousin, leaving that girl with a new guy and without her mate. I met my cousin still dancing with that girls mate that me and my cousin both already knew. I told um what had happened. She wanted to go find her mate so I took um to find her. The girl I had been kissing was now on her own. So we spoke to her. Then that guy came back held her hand, she said she was going back to his place. Off they went.
It all happened very fast so I am 90% she knew him.
They say its never the girls fault. What did I do wrong in this situation?

Later I was on the dance floor with just my cousin. This girl was dancing alone near us, giving me the eyes. So I turned and danced with her. She was another 7 at best. We started to dance and then kiss. My cousin wondered off and left me. After about ten mins of dancing, half chatting the best we could and kissing, I suggested we go upstairs to the smoking lounge to get some "fresh air", we could talk properly then too. As we got to the door which lead to the outside area, 3 girls came the other way that knew her and grabbed her and took her back the way I came. She never even looked back, so I thought to myself best not follow as that would be slightly creepy, but I never got her name, number or anything. To be honest, I wouldn't recognise her today if I bumped into her.
But as I said previously, its never the girls fault. But I still don't know what I could of done to of stayed with her longer in that situation.

Any thoughts guys or tips would be much appreciated!