When you started texting this girl was she dating someone already? That might be a problem. If a girl tells you that she really cares about you, but is unwilling to break up with her boyfriend, then she's not really into you.
If she had a boyfriend while she was professing to love you, then it sounds like she was just using you to meet her emotional needs. It's always ok to try get a girl to break up with her boyfriend to go out with you, but the girl better be worth the wrath of her boyfriend. I personally would not try to break up a girl that's already dating someone, unless I knew her really really well. And even then, there is a girl that comes to mind right now, that I am not trying to break up with her boyfriend. It seems sooo much easier just to find a different single girl to date.
If she told you that she's not interested, then that means that she is not interested. It's probably time to move on. There are a zillon other girls you can try to date. Expand your social circle. Really try to be more social. You will learn from every social interaction, even if you feel awkward about it.
Also, it's generally never a good idea to confess to a girl that you like her. Unless, you're absolutely positive that she's interested in you, that just puts a ton of pressure on her. She will normally say no. Instead, it's better to flirt a lot. Touch her appropriately. Make good eye contact. etc. Then ask her out. Normally, you should ask her in just a way that doesn't make it sound like a date.
Hey, you seem like a really fun person. Would you care to join me in some frisbee later today?
Don't use the word "date". If she shows up, then act like it's a date.
I remember high school being really tough. People make fun of you a lot. Keep trying. Life gets better after high school.