Being taken advantage

I have know this girl as a friend (P) for couple of years.The thing is that i fell in love with her, even though i already knew she was in a steady releationship for 2years even before when P and i first knew each other.At first it was secert because i was afraid of runing our friendship.Only a very few people knew about my feeling towards her,one of whom was her cousion.Whom she told P about my feeling,at first maybe she angry or shocked for whaterever reason some short of argument ensued between us.For couple of weeks she gave me the slient treatment, but for some reason she started to talk again.Even though we never brought up the subject again P knew that i have feelings for her.After the slient treatment was over i could sense a lot of difference in our friendship. She seem to accept what was going on but still she was having a good relationship. Maybe bcause it was love or for friendship, i deceided to back because i did not want to ruin her love life. But still i could sense difference between because if i did not call or meet her for more than a couple she will be angry and call me asking me why i did not call her and our conversation was not the same(i started to joke that i will be waiting if thing do not work out between P and her bf which she didi not mind). This did not used to happen before, she was also having some personal relationship problems of own.P was suspecting her boyfriend n seeing someone.They started have a lot of argument, but they managed to short it out.Altthough i was kinda of sad that i know i will not get the chance to be with her. For some reason i was ready to accept the fact that she will never belong to me.I overcome my jelousy and was ready to move on, and giving their relationship to blossom. But all things took a different turn, when i was out of town for just a week,but still a phone call away which i did almost daily. Not only she had broke off with her old boy friend but within a week she was with someone else. I had to hear this from her friends, it kinda broke me and i also felt insulted. I felt i was taken for garanted and taken advantage because she was turing to me help and other things. The thing really broke me was that fact (1)she started a new relationship within day of her old one (2)she did not told me anything and i have to hear that from someone (3)The fact she knew i loved her she still choice someone i m now so angry and felt insulted to the point like it is not worth speaking to her althoghter, although she still insists me on calling her and sometimes she still do ask me for some help what should i do???for some reason i still have feeling for her i can wait 4ever but i feel that i am now being taken advangate and taken for granted.