This girl has a boyfriend but flirts with me and I ignore her..Is it a good idea because I don't wanna lose her

There is this girl in my class, we are good friends and she constantly tries to get my attention. She is social (but not a flirter) and lively and I am completely opposite. Yet she always tries to talk to me, get my attention (also she texted me first and I even didn't have her number)

and acts very friendly. I responded positively and now we are very good friends. We texted daily and also we nick named each other. She showed all signs that she liked me but I never responded (intentionally). Then after a few days I got to know that she has a boyfriend(long distance) and so I eventually reduced texting her and talked to her only when she started the conversation. I had developed crush on her and wanted to get her out of my head, so I started calling her sis..(thinking that will put her off and she will ignore me)..But didn't go down well with her and she still showed signs that she liked me. So as a last option I have now completely started ignoring her (haven't talked or texted her for last 15 days even though we are in same class) . Now she looks sad and doesn't socialize as much as she did before and has become more of a serious girl (I used to tease her that I liked serious girls and not funny girls like her). But she still shows signs of interest (looks at me, tries to get my attention by talking with my buddies and gets enthusiastic when I talk to her(only about academics these days)).

I thought of her as an attention seeking girl and so I behaved in such a manner with her. But she is changed a lot now (because of me) and I feel it is hurting her. I never wanted to hurt her because she is one of my closest friends...But this "Crush or No Crush" had confused me a lot and now I am in terrible situation. I don't want to lose her as a friend and also is she really interested in me and should I ask her out..? (I really like her but she has a boyfriend from 3 years-for the last two years he is studying in another university and is in her contact only through phone).

plus I need help and some genuine advice.
This girl has a boyfriend but flirts with me and I ignore her..Is it a good idea because I don't wanna lose her
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