Ok here's my two cents. You my friend, need a wake up call. If this sounds harsh, bear in mind that I'm trying to help you.
You've realized that being a nice guy gets you nowhere with women. That's step 1. Step 2 is doing something about it. This is the step you seem to be stuck on, because you are afraid.
At the moment, you think that this girl is your only available option, and you feel that if you get rid of her, you will only make yourself more miserable because there is nothing else out there. THIS IS WRONG.
This girl is USING you as an EMOTIONAL TAMPON. No let me rephrase that, you are letting this girl use you. I'll explain what you are. Whenever this girl feels low about herself, or is having a bad day, she will be around you, giving you so called "mixed signals" because she knows you will always respond to her. By being there for her, and always listening to the b!tch, you are feeding her ego, and making her more emotionally stable. She is using you like a drug, to make herself feel better, to raise her self-esteem, and once it's up, she's gone, using her newly acquired self-esteem to attract another guy.
And you know what? You're the one who is to blame for all this. You're letting yourself get used, like a sucker. Jesus man, you're almost 20 years old, and you're still acting like a little boy about all this, it's time to grow up.
First of all, forget friendship. You will never have a real friendship with this b!tch. As soon as you start seeing her as just a friend and totally forget about dating her, she will start giving you signals again, then once you start responding to them, she will shut you down again.
Second of all, despite your own words, you don't like this girl. She uses you, she doesn't want you, yet as soon as you start moving on with your life, she drags you back into this drama even tho she is not sexually interested in you at all. This girl is eating away at your self-esteem, and is ruining your life. Let me repeat, you don't like her. You think you do, but in reality, you like the idea of who this girl could be, not who she is. All I can say is, stop fantasizing, because this girl wont change, at least not in the next few years. Who knows, maybe once she's 30+ and is a single, used up slut trying to find a nice guy like you to settle down with she will see the error of her ways (too late b!tch!). As it stands now, you can't change this girl, and you can't make her see you as anything more then an emotional tampon.
Your only option is to walk away from her, and never speak to her again. It's not gonna be easy, i'll tell you that. As soon as this girl realizes that you are not interested in her anymore, her self-esteem will plumet. After all, you are her lapdog, the guy that is supposed to follow her around and be infatuated with her even after she kicks you in the testicles day after day. When she realizes that she no longer has your attention, she will do anything to get you interested in her again. She will be nice and sweet, she will give you signal after signal. Don't fall for any of it. Just ignore her, don't give her your time of the day. Because as soon as you show interest in her again, she will use you up, and move on. So:
1.) Don't give her any attention. Talk to other girls, be friendly to everybody except her. If she tries to talk to you, be indifferent and tell her that you don't have time for this. Always find an excuse not to talk to her. Ignore her, no matter how sweet she becomes. You are hurting her ego by not being interested, and she wants to heal her ego, she doesn't give a damn about you.
Kick her out of your life.
2.) Read the DJ Bible. Don't read it all at once but read it day by day, and re-read it. Educate yourself about women and relationships. You will learn why women always treat you as a lapdog, instead of being attracted to you. You will learn how you can change this.
You know that old saying, give a man a fish, and you feed him for the day, but teach him how to fish, and you feed him for life? Well it's like that. All the writtings in the DJ Bible will teach you how to think properly, how to handle women, and how to solve your own problems. In short, it will teach you how to fish.
3.) Start putting the knowledge to use. You can know every little thing that attracts women, but if you don't have the balls to take the step and start talking to them, and practicing everything you learn, then you will never get anywhere.
You have to stop being shy, and introverted around women. Forget all that sh!t about being friends first, then lovers. If you don't let a girl know you're interested in her during the first 5 minutes of meeting her, she will always see you as just a friend. You have to be bold, you have to have balls, and you have to make the first move. It is your job to approach, it is your job to talk to her and make her comfortable, it is your job to ask for the number, it is your job to call her and arrange a date, it is your job to take it to the next level. You don't realize it yet, but you hold all the power in any relationship.
Good luck, and don't talk to this girl anymore.