Wats the best way proposing a lady who told me,she has a boyfriend already.?

i m 27 and she is 23.she has been a workmate since last year october when i started working there.i got interested in her the first time i saw her and i told her she was a nice girl which she seemed to like from the way she responded.i supose we became friends after a litle while and i asked her out for a movie of which she accepted.After a movie,we sat down for non alcoholic drinks and shared just a bit of each other`s back grounds for the first time and we also talked a litle about work matters.Then i proposed love by also stressing how i felt about her.she told me she had a boyfriend though she didnt stess much on that.After that she told me to give her time to think about it.A few days later,her friend was calling me in font of their departmental workmates that i was a boyfriend of that girl.But that girl was not around and the two friends work in the same department.When i asked her what she was talking about,she told me that her friend told her everything about her outing with me.i prettended by tryin to deny,knowing that i had also earlier shown interest in her.Anyway,i temporarily became happy uming that things were on the right truck.when i met my so called girlfriend a few days later,she didnt seem to be interested in talking to me.we had also earlier conversations on phone and i would say there didnt seem to be happness on her side.As time went on,she didnt even want to answer my calls.I told her friend everything and i asked her to help me out,and she agreed generously.The friend told me that,that girl had a boyfriend whom she loved so much but recieved nothing in return except for a chain of dissappointments.Despite everything,i continued trying to make her happy by offering to buy or do anything for her.Though shy,she has been accepting my offers.But there was a period when she hated me so much that she didnt even want to see me anywhere near her.Hence,i had also stopped talking to her because it was just difficult to talk to someone who didnt respond.Prior to that though a bit too late still,her friend had told me convincingly that i was going out with this other lady in my department of which i also convinced her that i didnt even had the phone number for the girl in quetion of which it was true.She even told me that everyone in their department new that the girl in quetion was my girfriend.Interestingly,she told me that the person who started the rumour was and is still our manager.that manager had information that i was ching after the girl i loved,unfortunately,he even told her the allegation.But recently,the girl seems to have changed the mind after i bought her a pair of pajamas on birthday which fell on april 12.She now talks to me though we dont really talk much but atleast she seems to be ready for a lenghthy descussion with me.Im not really sure how things are going with her boyfriend.But my wory is that,there is this other workmate guy whom she has been so close to such that some people including her friend think that the two are in love.At one of our lowest points,she told me that she had a boyfiend whom she loved so much and that her and i could only aford to be friends.But i still want her,please advise