Iftekhar, It is not at all wise to have sex with force with any woman at all! Be it your girlfriend, any woman with whom one is having a physical affair or even with one's own wife. It amounts to rape and one who has done it could be prosecuted under appropriate penal laws which could result in legal punishment also.
Apart from that don't you think its an offence against the humanity? Using force to get pleasure from the body of other person be it your lover, wife etc?
So as have stated in other answers try to win over her heart and soul. Do not long much for the physical pleasure(at least at this point of time) It could or would long last for the couple of minutes but if you would be able to touch her soul with your love,attachment, involvement she will remember you for whole of her life no matter you two get married or not!
So start to respect her feelings. Be a good listener and give it a point to do what she prefers to do or have. Try to become a very good friend of hers You can always use force to have sex with her. But it would only result in a life long resentment for you in her heart! So just leave this thought away from your mind. If she realizes your love for her she herself, one day would offer it to you and it would be most enjoyable that way only!
In the meantime also try to become a better humanbeing. Nurture some hobby, read books, listen to good music, watch good movies . or concentrate on your career and studies. Make yourself to a stature that no one could ignore you including her . She should feel pride for being your girl. That would be the MANly thing to do! NOT using force to have her! It would be very much unmanly and act of a cowardance ! Best of luck!