Is it lust, love or just a small crush (kinda long)?

there is this girl I'm totally infatuated with and have been since i met her about 6 years ago. I can't get her off my head. I love her looks and her personality. I told her I liked her a long time ago. she told me the same about a year and a half ago. Don't think its still true because she is going out with someone already and seems to be real happy with him. She is very guy friendly and has many others after her. we have been next door neighbors since we met. she is 2 years younger than me (me 17, her 15). I know she might not be the right 1 for me and that being like this will bring me alot of pain but I can't help it. I have tried many times to get over her and go after other girls. I have had other girls that I also like but none like her. all the times I tried to get over her, I get so close that I almost forget about her but then when I'm about ready to forget her, something just brings me back in and makes me like her even more then before. I flirt with her often but I'm also friendly so i won't get caught in the trap again but I still do. could it be that over time I could have fallen in love with her. I can't stop thinking about her. I have even had dreams about her. She is still too immature to see me as anything more then a boy she can mess around with or even use me but I can't let her go. what can I do to stop liking her. I know she isn't the right one for me. Maybe when she matures a bit I'll be able o do something but right now, its too soon.