Confusion

I have been in a relationship for the past 3 months now. Uptill now my relationship has been a smooth ride. Suddenly it has entered into a state of confusion. Actually my girlfriend works in the same office, we met and I was attracted towards her from day one. Eventually we started talking more and more and we decided to be together. She is 5 yrs younger to me aged 24 and I am 28. We have discussed about marriage, family and what not. Few days back, she heard stories about my past relationships in office and is forming an opinion that I cannot draw a line between frienship and relationship specially when it comes to a person of opposite sex. She feels, getting too close to a girl is one of my basic character and this will never change and because of this this, she can not trust me or says does not want to be a person who is failing to draw a line. We are still talking, still close and not too sure about her but I am madly in love with her. I am geneuine and from my end there's no lack of committment. I feel she also loves me but since the time she has formed an opinion, she is saying that although she loves me, however, she needs time to decide about our future. I have had one failed serious relationship in the past, where things were all set for marriage. Before I began my relationship with my present girlfriend, I told her all about my past. I am very much committed to this relationship and I am ready to give everything to make this a success. I want to get married to this girl. Now I am confused since she has sent me a message, where she mentioned that she needs time to think about our future. I am thinking too much since then and the very fear of loosing her is also making me nervous. I am trying to be normal, however, the whole thing is really affecting me. Could anyone suggest what should i do to make her believe that I am fully committed to this relationship?? Thanks. Nick.