Is she worth my attention?

I am talking to this girl that I had classes with in college. I have been talking to her for a year, not constantly just every now and then, I started getting a bit close to her by the end of this semester of college and we seemed to like talking with one another. I started hinting to her that I liked her and I finally gave up and told her that I did. She seemed thrilled a bit but sadly told me she has a bf who she has been with for 4 years now... I tried to get her attention lots by being the sweet kind guy I am, and she would seem to be all happy about it, but now that I am texting with her she doesnt seem to reply much to me and only uses short words, like yup, yes sir, etc.. which I am tired of.. I ask if I can call her but she always seems busy. I invite her out, ask her when she is available to go out, but refuses my offer even when I tell her to go out as friends... She seems to like the attention I am giving her but doesnt want to show it much? I dont know what to do.. She doesnt have anything about her bf on facebook or instagram, but she mentions little things about him to me nothing serious but yea. They tell me to not give up and reach the unreachable but I dunno sometimes I desperately feel like giving up...