I have been in a serious relationship for about 3 years. As I am going back to grad school we wanted to hold off marriage until I finished and just date exclusively until them (another 4 years). A few weeks ago we got in a huge fight. I am a really easy going, funny and carefree guy and she thinks I am too childish and we broke it off. Looking back at the three years, I have changed a lot I am more mature and serious now. Not dating her made me realize that the things I was doing were actually really childish, I am totally committed and serious about taking our relationship to a new level. We have hung out a few times since then, I have told her how I feel and that I would marry her and try to get into a instate school. On one of our dates we even went to "look" at engagement rings. She says she has an awesome time with me and loves going on dates with me, but she also says that she wants to date other guys and see what's out there.
My take on this is that:-
1. she is just dating me until she finds someone better
2. she will date a few guys and have to settle for me.
3. I also told her that dating 2 or 3 guys might not help as there are so many guys out there, so what am I suppose to do wait for years?

Any ideas on how I should handle this? I do not want to date other girls and I am totally serious about her. Thank you for you advice and sorry for a long question.