What to do when you really like a girl that already has boyfriend? ?

I dont know what to do. This girl seems like my perfect match. We first started as friends and she was goin to do date my best friend then he dumped her then I started talking to her as a friend I never ment to get attach to her even though she was phscially attractive and then I wanted to ask her to prom but I never maned up to it cuz of my friend so then another guy asked her and she went out wit him but he never really gave her much attention so she would always talk to me and I thought she talked to me to get advice from a friend then over summer we spoke like litterally 24/7 sometime we'd go to sleep to two a.m. Recently she sent me a pic that was for my eyes only and I actually flirted wit her and she had a new boy but when I ask her if she wants to hang out she is down. She nevers uses me because for the time that ive known her she has never asked me for anything even if i offer her something she just would say no. So it aint that so what is it. If it is attention she wants then why ia it when shes wit her boy sends me that stuff and still wants to hang out and I could count on her for everything. Idk but Im afraid that Im startin to fall for her and thats scary cuz i fear that i will be the one thats gonna end up wit a shatterd heart. Does she feel the same as me. Should I tell her? please i need help. I mess wit too many girls but I honestly would trade all of those to just have a chance to be wit her. None of them could ever amount to her. I try to hold it in but its now to a point where I cant bear it anymore. Please I need help