Some Advice concerning a girl?

First post on here, thought I could use some people's advice who are completely detached from the situation..

Posted on here for quite a long time under another alias, swore I'd never do this but well I just need someone else's thoughts on where to go from here. So without further ado..

I met this girl about 18months ago, started talking to her on facebook, got her number, texted her non stop etc etc. Asked her out last July, she said no, and friend zoned me effectively. So I moved on (Helped that she had no messages on her phone for 2months..) and I went to my school's formal (prom for any American's out there..) with this other girl who I got really close to, not romantically but just became really close friends. Whilst this was going on, I also had the first girls (Lets call her Sarah..) best friend talking to me pretty non stop, pretty constantly talking about Sarah. So naturally I'm pretty confused about this. Then her best friend asks me to go out with Jane, herself and another of their guys friends, I was bored so I went. Now this is where it gets interesting.. I get cornered by Saraj's sister, who basically says that my formal date (Lets call her Rachel..) is a *insert rude word* and that I should be concentrating more on Sarah, because and I quote "She really likes me." Now naturally this confuses the hell out of me. Anyway, turns out Sarah's sister goes in and tell her. I get a text from her, and suddenly I'm back to texting her pretty non stop with no thought to Rachel.

Things move on, I go out with her and her friends a few times, go to her birthday etc etc. Then after Christmas she starts dropping hints (The same way as she did in July..) that she likes me, I have her friend still going on about me "Omg you like her, I know you do, just ask her out" etc etc. I let this go for a few weeks then bite the bullet and ask her to go out in January, and she says yes. So we start going out, and it was all good. Then about a month ago she starts picking fights and making really subtle "digs" at me and I start arguing with her (Such to the extent that one Friday night she lets me get ready and then 10mins before we're due to go out and says "I'm not coming out tonight" Queue another weekend of arguing..) Then a week and a half ago she asks me to go out, says it's not working and I'm fine with that, but that I can't go on speaking to her the way I have been and so I think we should just stop talking (Childish I know but I was hurt.) She was in agreement with this, we said bye and I thought that was fun while it lasted.

However.. She texted me that same day, saying she still really liked me, and her best friend tells me she was in pieces. So I'm like, right, okay. Then I tell her we need to talk about what we're going to do, seeing as it seemed to be hurting her big time not talking to me (I still cared about her..) so we arrange to go out that Saturday. We go out and chat, get everything out in the open, and she basically says "I don't see why in a few weeks we can't get back together.I'm not saying we will but." Which is fair enough, I was happy with that. Her best friend is still saying she thinks we'll get back together etc. She texted me the other day saying "I still really like, depend on you or something sometimes.." But nothing has really changed, she's constantly texting, but yet I don't get the impression anything will change should we get back together.

Some advice please? I appreciate that none of that probably makes any sense.. I just needed to get it out somewhere. Thanks in advance.

For the record, I am only 18, I'm not taking this to be the end of my life, I just wanted to get it out somewhere and maybe get some help as to what I should do.

This may sound funny because I'm giving you advice but stop listening to what people are telling you to do. You are 18 and should know by now what YOU want and not go by what everyone else wants. Take a step back and assess the situation in regards to YOU and make the best decision.

To put it bluntly, she's a childish game-playing control freak. I'd steer clear until and unless she grows up, and probably even then.

So how is it dating 2 girls at once? This "best friend" really needs to get a life.
I get it, you're 18.. but c'mon man... You're letting this girl dictate the direction of your relationship.
She realized you were talking to another girl and brought the attention back on her.. She decided you might be able to get back together in a few weeks?
and whenever she feels you slipping its. "i like, depend on your or something."

I usually don't say this but in this case you might be better off ditching it.

I've been hearing that alot, but how do I go about letting her go gently without leaving things on "bad terms"? I still care about her, she's been my friend for 18months after all..

There is no way of no leaving on bad terms no matter how you put it to her. When she finally grows up, which could be years from now, she will understand then. Sorry it's just one of life's lessons we all have gone through.